IM UNIT 2 Equations and Inequalities Teacher Guide

Here is an inequality:
Select all the values of that are solutions to the inequality.

A.
B.
C.
D.
E.
F.

Solution

["A", "B", "C", "D"]

(From Unit 2, Lesson 19.)

Algebra1 Unit 2 Lesson 22 CC BY 2019 by Illustrative Mathematics 33

8. Problem 8

Statement .

Here is a graph of the equation

a. Are the points and solutions to

the equation? Explain or show how you know.

b. Check if each of these points is a solution to

the inequality :

c. Shade the solutions to the inequality.

d. Are the points on the line included in the
solution region? Explain how you know.

Algebra1 Unit 2 Lesson 22 CC BY 2019 by Illustrative Mathematics 34

Solution

a. The first one is not a solution but the second one is
a solution. I substituted the values into the
equation to see whether or not they were true.

b. and are solutions. and

are not solutions.

c. See graph.

d. They are included because they are the values

where is equal to -8 so they are included in

the solution to the inequality .

(From Unit 2, Lesson 21.)

Algebra1 Unit 2 Lesson 22 CC BY 2019 by Illustrative Mathematics 35

Lesson 23: Solving Problems with Inequalities
in Two Variables

Goals
• Identify an inequality, a graph, an ordered pair, and a description that represent the

constraints and possible solutions in a situation.

• Understand that a constraint on two variables can be represented by an inequality, a

graph (a half-plane), and a verbal description.

• Write inequalities in two variables to represent the constraints in a situation and use

technology to graph the solution set to answer questions about the situation.

Learning Targets
• I can use graphing technology to find the solution to a two-variable inequality.

• When given inequalities, graphs, and descriptions that represent the constraints in a

situation, I can connect the different representations and interpret them in terms of
the situation.

Lesson Narrative

By now students recognize that solutions to linear inequalities can be found by graphing,
and that this can be done by first graphing a related equation and deciding on the solution
region. In this lesson, they learn to use graphing technology to find the solution set of a
linear inequality in two variables.

Students then use this skill to solve problems that involve inequalities. They write linear
inequalities to represent the constraints in situations and then use the representations
(including the graphs of the solutions) to answer questions about the situations. As they
write inequalities from descriptions, decide on the solution sets, and interpret points in a
solution region, students engage in quantitative and abstract reasoning (MP2).

Alignments
Addressing

• HSA-CED.A.3: Represent constraints by equations or inequalities, and by systems of

equations and/or inequalities, and interpret solutions as viable or nonviable options
in a modeling context. For example, represent inequalities describing nutritional and
cost constraints on combinations of different foods.

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 1

• HSA-REI.D.12: Graph the solutions to a linear inequality in two variables as a

half-plane (excluding the boundary in the case of a strict inequality), and graph the
solution set to a system of linear inequalities in two variables as the intersection of
the corresponding half-planes.

Instructional Routines

• Aspects of Mathematical Modeling
• Card Sort
• Graph It
• MLR7: Compare and Connect
• Take Turns

Required Materials
Graphing technology
Examples of graphing technology are: a handheld graphing calculator, a computer with a
graphing calculator application installed, and an internet-enabled device with access to a
site like desmos.com/calculator or geogebra.org/graphing. For students using the digital
materials, a separate graphing calculator tool isn't necessary; interactive applets are
embedded throughout, and a graphing calculator tool is accessible on the student digital
toolkit page.

Pre-printed slips, cut from copies of the blackline master

Required Preparation
Print and cut up slips from the blackline master. Prepare 1 copy for every 2 students.

Acquire devices that can run Desmos (recommended) or other graphing technology. It is
ideal if each student has their own device. (If students typically access the digital version of
the materials, Desmos is always available under Math Tools.)

Student Learning Goals

• Let’s practice writing, interpreting, and graphing solutions to inequalities in

two variables.

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 2

23.1 Graphing Inequalities with Technology

Warm Up: 10 minutes
In this warm-up, students use graphing technology to graph simple linear inequalities in
two variables. They practice adjusting the graphing window until the solution regions
become visible and give useful information. Later in the lesson, students will write
inequalities that represent constraints in different situations and find the solution sets.
The exercises here prepare students to do the latter using graphing technology.

Addressing

• HSA-REI.D.12

Instructional Routines

• Graph It

Launch

Give students access to graphing technology. If using Desmos:

• Explain to students that typing " " gives the symbol and typing " " gives the

symbol.

• Remind students that the and buttons can be used to zoom in and out of the

graphing window, and that the wrench button in the upper right corner can be used

to set the graphing window precisely.

If using other graphing technology available in your classroom:

• Demonstrate how to enter the and symbols.

• Remind students how to set a useful graphing window by zooming in or out, and how

to set a precise graphing window by specifying the horizontal and vertical
boundaries.

• (For technology that takes only equations or inequalities in slope-intercept form:)

Remind students that some inequalities might need to be rewritten such that is
isolated before the inequality can be entered into the graphing tool.

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 3

Student Task Statement

Use graphing technology to graph the solution region of each inequality and sketch
each graph. Adjust the graphing window as needed to show meaningful
information.

Student Response
Sample graphs:

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 4

Activity Synthesis
Display the correct solution regions for all to see and ask students to check their graphs.
Discuss any challenges students may have come across when trying to graph using
technology.

Explain to students that they will now use graphing technology to find solutions to some
inequalities that represent constraints in situations.

23.2 Solving Problems with Inequalities in Two
Variables

25 minutes
This activity enables students to integrate several ideas and skills from the past few
lessons. Students write inequalities in two variables to represent constraints in situations,
use technology to graph the solutions, interpret points in the solution regions, and use the
inequalities and the graphs to answer contextual questions. In doing so, they engage in
aspects of mathematical modeling (MP4).

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 5

The questions in this activity are written in pairs. The same constraints and contexts will be
used in an upcoming lesson on systems of linear inequalities in two variables.

Decide on the structure for the activity depending on the time available and the amount of
practice each student needs. Here are some possibilities:

• Assigning all three pairs of questions to all students.
• Assigning each student a pair of questions, arranging students who work on different

pairs in groups of 3, and asking them to explain their solutions to one another.

• Arranging students in groups of 3, assigning the same pair of questions to each

group, and—for each pair of questions—asking one group to present the solutions to
the class.

Addressing

• HSA-CED.A.3
• HSA-REI.D.12

Instructional Routines

• Aspects of Mathematical Modeling
• Graph It

Launch
Continue to provide access to graphing technology. Explain that students will now write
and graph inequalities to solve problems about some situations.

Assign at least one pair of questions about the same context to each student. See Activity
Narrative for some possible ways to structure the activity.

Some students might not be familiar with terms such as "savings," "checking," or
"premium." Explain any unfamiliar terms as needed.

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 6

Support for Students with Disabilities

Engagement: Provide Access by Recruiting Interest. Leverage choice around perceived
challenge. Chunking this task into more manageable parts may also support students
who benefit from additional processing time. Invite students to select one of the
situations to work on. If students are arranged in groups, each member should be
prepared to share during the whole-class discussion.
Supports accessibility for: Attention; Social-emotional skills

Anticipated Misconceptions

One inequality in the bank account context involves only one variable. Some students

might think that all inequalities they write must include two variables. Reassure them that

this might not always be the case. Consider pointing to examples from earlier activities in

which they wrote or graphed inequalities such as or .

Student Task Statement
Here are three situations. There are two questions about each situation. For each
question that you work on:

a. Write an inequality to describe the constraints. Specify what each variable
represents.

b. Use graphing technology to graph the inequality. Sketch the solution region on
the coordinate plane and label the axes.

c. Name one solution to the inequality and explain what it represents in that
situation.

d. Answer the question about the situation.

Bank Accounts

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 7

1. A customer opens a checking account and a
savings account at a bank. They will deposit a
maximum of $600, some in the checking account
and some in the savings account. (They might not
deposit all of it and keep some of the money as
cash.)

If the customer deposits $200 in their checking
account, what can you say about the amount they
deposit in their savings account?

2. The bank requires a minimum balance of $50 in
the savings account. It does not matter how much
money is kept in the checking account.

If the customer deposits no money in the checking
account but is able to maintain both accounts
without penalty, what can you say about the
amount deposited in the savings account?

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 8

Concert Tickets
1. Two kinds of tickets to an outdoor concert were
sold: lawn tickets and seat tickets. Fewer than 400
tickets in total were sold.
If you know that exactly 100 lawn tickets were
sold, what can you say about the number of seat
tickets?

2. Lawn tickets cost $30 each and seat tickets cost
$50 each. The organizers want to make at least
$14,000 from ticket sales.
If you know that exactly 200 seat tickets were sold,
what can you say about the number of lawn
tickets?

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 9

Advertising Packages

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 10

1. An advertising agency offers two packages for
small businesses who need advertising services. A
basic package includes only design services. A
premium package includes design and promotion.
The agency's goal is to sell at least 60 packages in
total.
If the agency sells exactly 45 basic packages, what
can you say about the number of premium
packages it needs to sell to meet its goal?

2. The basic advertising package has a value of
$1,000 and the premium package has a value of
$2,500. The goal of the agency is to sell more than
$60,000 worth of small-business advertising
packages.
If you know that exactly 10 premium packages
were sold, what can you say about the number of
basic packages the agency needs to sell to meet its
goal?

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 11

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 12

Student Response
Bank Accounts

1. Sample response:
a. , where represents the amount
deposited in checking account and the amount
deposited in savings account.

b. See graph.

c. . $500 is deposited in checking and $100
in saving. The total is $600, which is the maximum
amount the customer plans to deposit.

d. The customer deposits $400 or less in savings.

2. Sample response:
a. , where is the balance in the savings
account.

b. See graph.

c. 75. The customer deposits $75 in savings, which is
more than $50.

d. The customer deposits at least $50 in savings
account.

Concert Tickets

1. Sample response:

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 13

a. , where represents the number of
lawn tickets and represents the number of seat
tickets.

b. See graph.

c. . 100 lawn tickets and 200 seat tickets
were sold. The total is 300 tickets, which is fewer
than 400.

d. Fewer than 300 seat tickets were sold.

2. Sample response: represents the
a. , where represents the
number of lawn tickets and
number of seat tickets.

b. See graph.

c. 100 lawn tickets and 250 seat tickets. The ticket
sales would be 15,000, which is more than 14,000.

d. At least 134 lawn tickets must be sold to make at
least $14,000.

Advertising Packages

1. Sample response:
a. , where represents the number of
basic packages and represents the number of
premium packages.

b. See graph.

c. . 50 basic packages and 30 premium ones
are sold. The total is 80, which is more than 60.

d. The agency needs to sell at least 15
premium packages to meet its goal.

2. Sample response:

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 14

a. , where represents the
number of basic packages and represents the
number of premium packages.

b. See graph.

c. . 10 basic packages and 30
premium packages are sold. The total value is
$85,000, which is more than $60,000.

d. The agency needs to sell more than 20 premium
packages.

Are You Ready for More?
This activity will require a partner.

1. Without letting your partner see it, write an equation of a line so that both the

-intercept and the -intercept are each between -3 and 3. Graph your

equation on one of the coordinate systems.

Your Inequality Your Partner's Inequality

2. Still without letting your partner see it, write an inequality for which your
equation is the related equation. In other words, your line should be the
boundary between solutions and non-solutions. Shade the solutions on your
graph.

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 15

3. Take turns stating coordinates of points. Your partner will tell you whether
your guess is a solution to their inequality. After each partner has stated a
point, each may guess what the other’s inequality is. If neither guesses
correctly, play continues. Use the other coordinate system to keep track of
your guesses.

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 16

Student Response
Answers vary.

Activity Synthesis
Select one student or one group to present their solutions for each pair of questions.
Focus the discussion on how students used the inequalities and graphs to help answer the
question about each situation.

Students who worked on the same questions might end up with different graphs and
answers because they wrote different inequalities (which might not correctly represent the
constraints in the situation). If this happens, analyze the different inequalities and look for
the potential causes for the discrepancy. One possibility is that the wrong symbols or the
wrong numbers were entered into the graphing tool.

Another possibility might be that students made different decisions about the quantities
being assigned to the vertical and horizontal axes. In that case, both versions of the graphs
might be correct, but the answers to questions might be different if one of the graphs is
not interpreted correctly.

23.3 Card Sort: Representations of Inequalities

Optional: 15 minutes
This optional activity allows students to practice interpreting inequalities in context and
reasoning about their solutions graphically and numerically. A sorting and matching task
gives students opportunities to analyze representations, statements, and structures
closely and to make connections (MP2, MP7).

As students explain their thinking to a partner, encourage them to use precise language
and mathematical terms to refine their explanations (MP6).

Here are images of the sorted cards for reference and planning.

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 17

Addressing

• HSA-CED.A.3
• HSA-REI.D.12

Instructional Routines

• Card Sort
• MLR7: Compare and Connect
• Take Turns

Launch
Arrange students in groups of 2. Distribute one set of pre-cut slips or cards to each group.

Ask students to take turns matching a group of 4 cards with representations of the same
situation and explaining how they know the representations belong together. Emphasize
that while one partner explains, the other should listen carefully, and the group should
discuss any disagreements.

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 18

Support for English Language Learners

Representing, Conversing: MLR7 Compare and Connect. Use this routine to prepare
students for the whole-class discussion. At the appropriate time, invite groups to
create a visual display for one of the situations. In addition to the 4 cards, students
should consider what types of details (annotations, notes, arrows, etc.) to include on
their displays that will help communicate the reasoning behind their choices. Begin
the whole-class discussion by selecting and arranging 2–4 displays for all to see. Give
students 2–3 minutes of quiet think time to interpret the displays before inviting the
authors to present their work.
Design Principle(s): Optimize output; Cultivate conversation

Support for Students with Disabilities

Engagement: Develop Effort and Persistence. Provide a checklist that focuses on
increasing the length of on-task orientation in the face of distractions. For example,
use the directions to create a bulleted checklist of the steps. Invite students to check
off the steps at the completion of each phase of the activity. An example list may
include items such as: taking turns with matching, 2 explanations for each partner, 2
responses/rebuttals for each partner, and 4 agreed upon final responses as a team.
Supports accessibility for: Attention; Social-emotional skills

Student Task Statement
Your teacher will give you a set of cards. Take turns with your partner to match a
group of 4 cards that contain: a situation, an inequality that represents it, a graph
that represents the solution region, and a solution written as a coordinate pair.

For each match that you find, explain to your partner how you know it’s a match.

For each match that your partner finds, listen carefully to their explanation. If you
disagree, discuss your thinking and work to reach an agreement.

Record your matches.

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 19

Group 1 Group 2

• situation: perimeter of a rectangle • situation: jar of coins

• inequality: • inequality:

• a solution: • a solution:

• sketch of graph: • sketch of graph:

Group 3 Group 4

• situation: honey and jam • situation: a school trip
• inequality: • inequality:
• a solution: • a solution:
• sketch of graph: • sketch of graph:

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 20

Student Response
See images of sorted cards in the Activity Narrative.

Activity Synthesis
Select groups of students to share their results and explain their rationales. After a group
explains why they believe a set of cards belong together, ask if other groups reasoned
about the matches the same way or if they approached the matching differently.

Attend to the language that students use in their explanations by giving them
opportunities to describe the inequalities, graphs, or solutions more precisely.

Lesson Synthesis

Summarize the lesson by discussing students' work for the last activity (about bank
accounts, concert tickets, and advertising packages) and inviting them to reflect on their
reasoning process. Discuss questions such as:

• "Of the four things you were asked to do in the last activity—writing an inequality,

graphing the solutions, identifying and interpreting a particular solution, and
answering the question about the situation—which one did you find most
challenging or prone to error?"

• "How is graphing linear inequalities using technology similar to graphing them by

hand? How is it different?"

• "You have previously used technology to graph linear equations in two variables.

How is graphing linear inequalities in two variables different than graphing
equations?"

23.4 The Band Played On

Cool Down: 5 minutes
Addressing

• HSA-CED.A.3
• HSA-REI.D.12

Launch
Provide access to graphing technology.

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 21

Student Task Statement
A band is playing at an auditorium with floor seats and balcony seats. The band
wants to sell the floor tickets for $15 each and balcony tickets for $12 each. They
want to make at least $3,000 in ticket sales.

1. How much money will they collect for selling floor tickets?

2. How much money will they collect for selling balcony tickets?

3. Write an inequality whose solutions are the number of floor and balcony
tickets sold if they make at least $3,000 in ticket sales.

4. Use technology to graph the solutions to your inequality, and sketch the
graph.

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 22

Student Response
1.
2.
3.
4. See graph.

Student Lesson Summary . We
.
Suppose we want to find the solution to
can start by graphing the related equation

When identifying the solution region, it is important not
to assume that the solution will be above the line
because of a “>” symbol or below the line because of a
“<” symbol.

Instead, test the points on the line and on either side of
the line and see if they are solutions.

For , points on the line and above the line are not solutions to the

inequality because the pairs make the inequality false. Points that are below

the lines are solutions, so we can shade that lower region.

Graphing technology can help us graph the solution to an inequality in two
variables.

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 23

Many graphing tools allow us to enter inequalities such
as and will show the solution region, as
shown here.

Some tools, however, may require the inequalities to be
in slope-intercept form or another form before
displaying the solution region. Be sure to learn how to
use the graphing technology available in your
classroom.

Although graphing using technology is efficient, we still need to analyze the graph
with care. Here are some things to consider:

• The graphing window. If the graphing window is

too small, we may not be able to really see the
solution region or the boundary line, as shown
here.

• The meaning of solution points in the situation.

For example, if and represent the lengths of
two sides of a rectangle, then only positive values
of and (or points in the first quadrant) make
sense in the situation.

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 24

Lesson 23 Practice Problems

1. Problem 1

Statement

This year, students in the 9th grade are collecting dimes and quarters for a
school fundraiser. They are trying to collect more money than the students
who were in the 9th grade last year. The students in 9th grade last year
collected $143.88.
Using to represent the number of dimes collected and to represent the
number of quarters, which statement best represents this situation?

A.
B.
C.
D.

Solution

D

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 25

2. Problem 2

Statement

A farmer is creating a budget for planting soybeans and wheat. Planting
soybeans costs $200 per acre and planting wheat costs $500 per acre. He
wants to spend no more than $100,000 planting soybeans and wheat.

a. Write an inequality to describe the constraints. Specify what each
variable represents.

b. Name one solution to the inequality and explain what it represents in
that situation.

Solution

Sample responses:

a. where is the number of acres of soybeans and is
the number of acres of wheat.

b. One solution is , which means the farmer plants 40 acres of soybeans

and 100 acres of wheat and spends $58,000.

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 26

3. Problem 3

Statement

Elena is ordering dried chili peppers and corn husks
for her cooking class. Chili peppers cost $16.95 per
pound and corn husks cost $6.49 per pound.
Elena spends less than $50 on pounds of dried
chili peppers and pounds of corn husks.
Here is a graph that represents this situation.

a. Write an inequality that represents this situation.

b. Can Elena purchase 2 pounds of dried chili peppers and 4 pounds of corn
husks and spend less than $50? Explain your reasoning.

c. Can Elena purchase 1.5 pounds of dried chili peppers and 3 pounds of
corn husks and spend less than $50? Explain your reasoning.

Solution is not part of the
is part of the
Sample responses:

a.

b. No, she will spend more than $50 because the point
solution region shown in the graph.

c. Yes, she will spend less than $50 because the point
solution region shown in the graph.

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 27

4. Problem 4
Statement

Which inequality is represented by the graph?

A.
B.
C.
D.

Solution

B

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 28

5. Problem 5

Statement ◦ mean: 7.3
◦ median: 7.1
Here are some statistics for the ◦ standard deviation: 1.1
number of minutes it took each of 9 ◦ Q1: 6.8
members of the track team to run 1 ◦ Q3: 7.4
mile.

When a tenth time is added to the list,
the standard deviation increases to
1.5. Is the tenth time likely an outlier?
Explain your reasoning.

Solution

Sample response: To be an outlier, the time would have to be less than 5.9 minutes
or greater than 8.3 minutes. If the new value is between 5.9 minutes and 8.3 minutes,
then it differs by around 1 standard deviation from the mean. Adding such a value
should not increase the standard deviation, certainly not by a large amount as has
happened here. This means that the new value is probably an outlier.

(From Unit 1, Lesson 14.)

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 29

6. Problem 6
Statement

Elena is solving this system of equations:

She multiplies the second equation by 2, then subtracts the resulting

equation from the first. To her surprise, she gets the equation .

What is special about this system of equations? Why does she get this result
and what does it mean about the solutions? (If you are not sure, try graphing
them.)

Solution

Sample responses: The two equations are multiples of each other and are equivalent.
Equivalent equations are represented by the same graph and have the same
solutions as one another. This system has infinitely many solutions.

(From Unit 2, Lesson 17.)

7. Problem 7

Statement

Jada has a sleeping bag that is rated for F. This means that if the
temperature outside is at least F, Jada will be able to stay warm in her
sleeping bag.

a. Write an inequality that represents the outdoor temperature at which
Jada will be able to stay warm in her sleeping bag.

b. Write an inequality that represents the outdoor temperature at which a
thicker or warmer sleeping bag would be needed to keep Jada warm.

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 30

Solution ?

a. , where is temperature in degrees Fahrenheit.
b.

(From Unit 2, Lesson 18.)

8. Problem 8

Statement

What is the solution set to this inequality:

Solution

(From Unit 2, Lesson 19.)

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 31

9. Problem 9 .

Statement

Here is a graph of the equation

a. Are the points and solutions to the equation? Explain or

show how you know.

b. Check if each of these points is a solution to the inequality :

c. Shade the solutions to the inequality .

d. Are the points on the line included in the solution region? Explain how
you know.

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 32

Solution

a. is a solution but is not a solution. I

substituted the values into the equation to see

whether or not they were true.

b. and are solutions. and
are not solutions.

c. See graph.

d. They are not included. Because they are the values

where is equal to 15, they are not included

in the solution to the inequality .

(From Unit 2, Lesson 21.)

10. Problem 10

Statement

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 33

A store sells notepads in packages of 24 and packages of 6. The organizers of a
conference needs to prepare at least 200 notepads for the event.

a. Would they have enough notepads if they bought these quantities?
i. Seven packages of 24 and one package of 6

ii. Five packages of 24 and fifteen packages of 6

b. Write an inequality to represent the relationship between the number of
large and small packages of notepads and the number of notepads
needed for the event.

c. Use graphing technology to graph the solution set to the inequality.
Then, use the graph to name two other possible combinations of large
and small packages of notepads that will meet the number of notepads
needed for the event.

Solution

a. i. No, this is 174 notepads, which is fewer than 200.
ii. Yes, this is 210 notepads, which is greater than 200.

b.
c. Sample response: 8 large packages and 10 small packages, and 5 large packages

and 18 small packages

(From Unit 2, Lesson 22.)

Algebra1 Unit 2 Lesson 23 CC BY 2019 by Illustrative Mathematics 34

Lesson 24: Solutions to Systems of Linear
Inequalities in Two Variables

Goals
• Given descriptions and graphs that represent two constraints, identify values that

satisfy each constraint and those that satisfy both constraints simultaneously.

• Understand that the solution set of a system of inequalities in two variables is

comprised of any pair of values that make both inequalities true, and that it is
represented graphically by the region where the graphs overlap.

• Write systems of inequalities in two variables, use technology to graph the solutions,

and interpret the solutions in context.

Learning Targets
• I can write a system of inequalities to describe a situation, find the solution by

graphing, and interpret points in the solution.

• I know what is meant by "the solutions to a system of inequalities" and can describe

the graphs that represent the solutions.

• When given descriptions and graphs that represent two different constraints, I can

find values that satisfy each constraint individually, and values that satisfy both
constraints at once.

Lesson Narrative

Earlier in the unit, students solved systems of linear equations in two variables. They also
found solutions to linear inequalities in two variables. In this lesson, students build on
those understandings to find the solutions to systems of linear inequalities in two
variables.

Students learn that two linear inequalities that represent the constraints in the same
situation form a system of inequalities, and that the solutions to the system include all
numbers that satisfy both constraints simultaneously. Graphically, the solution set of the
system is represented by the region where the graphs of the individual inequalities
overlap.

Throughout the lesson, students endeavor to find values that satisfy multiple constraints,
both in terms of a situation and in absence of one. The process requires that students

Algebra1 Unit 2 Lesson 24 CC BY 2019 by Illustrative Mathematics 1

make sense of problems and persevere in solving them (MP1) and to reason quantitatively
and abstractly (MP2).

Alignments
Building On

• HSA-REI.C.6: Solve systems of linear equations exactly and approximately (e.g., with

graphs), focusing on pairs of linear equations in two variables.

Addressing

• HSA-CED.A.3: Represent constraints by equations or inequalities, and by systems of

equations and/or inequalities, and interpret solutions as viable or nonviable options
in a modeling context. For example, represent inequalities describing nutritional and
cost constraints on combinations of different foods.

• HSA-REI.D.12: Graph the solutions to a linear inequality in two variables as a

half-plane (excluding the boundary in the case of a strict inequality), and graph the
solution set to a system of linear inequalities in two variables as the intersection of
the corresponding half-planes.

Building Towards

• HSA-REI.D.12: Graph the solutions to a linear inequality in two variables as a

half-plane (excluding the boundary in the case of a strict inequality), and graph the
solution set to a system of linear inequalities in two variables as the intersection of
the corresponding half-planes.

Instructional Routines

• Anticipate, Monitor, Select, Sequence, Connect

• Graph It

• MLR8: Discussion Supports

Required Materials

Graphing technology
Examples of graphing technology are: a handheld graphing calculator, a computer with a
graphing calculator application installed, and an internet-enabled device with access to a
site like desmos.com/calculator or geogebra.org/graphing. For students using the digital
materials, a separate graphing calculator tool isn't necessary; interactive applets are
embedded throughout, and a graphing calculator tool is accessible on the student digital
toolkit page.

Algebra1 Unit 2 Lesson 24 CC BY 2019 by Illustrative Mathematics 2

Required Preparation
Acquire devices that can run Desmos (recommended) or other graphing technology. It is
ideal if each student has their own device. (If students typically access the digital version of
the materials, Desmos is available under Math Tools.)

Be prepared to display an embedded applet for all to see.

Student Learning Goals

• Let’s look at situations where two constraints (that can be expressed

by inequalities) must be met simultaneously.

24.1 A Silly Riddle

Warm Up: 10 minutes
This warm-up reminds students about systems of equations and their solutions. Students
recall that a solution to a linear equation in two variables is any pair of numbers that
makes the equation true, and that a solution to a system of two equations in two variables
is a pair of numbers that make both equations true.

The given system has a solution that is hard to find mentally, but can be calculated
algebraically or by using graphing technology.

Making graphing technology available gives students an opportunity to choose
appropriate tools strategically (MP5).

Building On

• HSA-REI.C.6

Building Towards

• HSA-REI.D.12

Instructional Routines

• Graph It

Anticipated Misconceptions

Students who graph the system of equations using technology may estimate from the

graph and offer as a solution. Ask them to check whether really does equal

5.678.

Algebra1 Unit 2 Lesson 24 CC BY 2019 by Illustrative Mathematics 3

Student Task Statement
Here is a riddle: “I am thinking of two numbers that add up to 5.678. The difference
between them is 9.876. What are the two numbers?”

1. Name any pair of numbers whose sum is 5.678.
2. Name any pair of numbers whose difference is 9.876.
3. The riddle can be represented with two equations. Write the equations.

4. Solve the riddle. Explain or show your reasoning.

Student Response
1. Sample response: 2 and 3.678.

2. Sample response: 10 and 0.124.

3. Let and represent the numbers: and
.

4. The two numbers are 7.777 and -2.099. Sample

reasoning:

◦ Graphing the two equations gives two lines that

intersect at .

◦ Adding the two equations eliminates the

-variable and gives or .

Substituting this -value into either equation gives

.

Activity Synthesis

Ask students using different methods to briefly describe their solving process. Record and
display their reasoning (including a graph) for all to see.

Algebra1 Unit 2 Lesson 24 CC BY 2019 by Illustrative Mathematics 4

If not mentioned in students' explanations, highlight that the riddle can be solved by
writing and solving a system of equations. Each equation represents a constraint. Ask
students:

• "What constraints do the two equations represent?" (The first equation represents a

constraint about the sum of the two numbers. The second represents a constraint
about the difference of the two numbers.)

• "What does a solution to the system represent?" (The solution is a pair of numbers

that simultaneously meet both constraints or make both equations true.)

• "How many pairs of numbers meet both constraints at the same time?" (Only one

pair. The graphs of the equations intersect at one point.)

24.2 A Quilting Project

15 minutes
In this activity, students encounter a situation in which two constraints that can be
expressed with inequalities are represented on two separate graphs, which makes it
challenging to find pairs of values that meet both constraints simultaneously. This
motivates a desire to represent both constraints on the same graph.

As students work, monitor for the different ways students try to find a pair of values that
satisfy both inequalities. Some likely approaches:

• Substituting different - and -values into the inequalities until they find a pair that

make both inequalities true.

• Visually estimating a point in the solution region of one graph that appears likely to

also be in the solution region of the other graph (for example, noticing that is in

the solution region of each inequality.

• Graphing both inequalities on the same coordinate plane (either using technology or

by copying the line in one graph onto the other graph) and finding a point in the

region where they overlap.

Identify students who use these or other approaches and ask them to share during class
discussion.

Making graphing technology available gives students an opportunity to choose
appropriate tools strategically (MP5).

Algebra1 Unit 2 Lesson 24 CC BY 2019 by Illustrative Mathematics 5

Addressing

• HSA-REI.D.12

Instructional Routines

• Anticipate, Monitor, Select, Sequence, Connect

• Graph It

Launch
Give students a moment to read the task statement and look at the two graphs. Ask
students:

• “What are the two constraints in this situation?” (A length constraint and a cost

constraint)

• “Which graph represents which constraint? How do you know?” (The first graph

represents the length constraint. Possible explanations:

◦ The graph intersects the vertical and horizontal axes at approximately 9.5,

which means that if the quilter bought 0 yards of one color, he will need at least
9.5 yards of the other color.

◦ The length constraint says "at least 9.5 yards," so the lengths must include

values greater than 9.5, which is shown by the shaded region of the first graph.

◦ The second graph represents the cost constraint. If the quilter bought 0 yards of

the light color, he could buy up to , or about 8.5, yards of the dark color
fabric. This corresponds to the vertical intercept of the second graph.)

◦ The cost constraint says "up to $110," so the lengths must be below certain

limits. The solution region of the second graph shows values below a boundary.)

Arrange students in groups of 2 and provide access to graphing technology, in case
requested.

Ask students to pause briefly after they have written both inequalities. Verify that
the inequalities that students have written accurately represent the constraints before
they proceed to the rest of the activity.

Algebra1 Unit 2 Lesson 24 CC BY 2019 by Illustrative Mathematics 6

Support for Students with Disabilities

Engagement: Develop Effort and Persistence. Encourage and support opportunities for
peer interactions. During the time spent on the two questions in the launch, invite
students to brainstorm with a partner before sharing with the whole-class. Display
sentence frames that elicit descriptive observations: “I notice that. . .”, as well as
frames that support interpretation and representation “_____ represents _____
because . . . ”.
Supports accessibility for: Language; Social-emotional skills

Student Task Statement
To make a quilt, a quilter is buying fabric in two colors,
light and dark. He needs at least 9.5 yards of fabric in
total.

The light color costs $9 a yard. The dark color costs
$13 a yard. The quilter can spend up to $110 on fabric.

Here are two graphs that represent the two constraints.

AB

1. Write an inequality to represent the length constraint. Let represent the
yards of light fabric and represent the yards of dark fabric.

Algebra1 Unit 2 Lesson 24 CC BY 2019 by Illustrative Mathematics 7

2. Select all the pairs that satisfy the length constraint.

3. Write an inequality to represent the cost constraint.
4. Select all the pairs that satisfy the cost constraint.

5. Explain why satisfies the cost constraint, but not the length constraint.

6. Find at least one pair of numbers that satisfies both constraints. Be prepared
to explain how you know.

7. What does the pair of numbers represent in this situation?

Student Response
1. or equivalent

2. , , and

3. or equivalent

4. , , and

5. 2 yards of each color costs $44, which is less than $110, so it satisfies this constraint.
However, this is only 4 yards altogether, which is not at least 9.5 yards.

6. Sample responses: , , and . (Any point that is in the intersection

of the two inequalities.)

Algebra1 Unit 2 Lesson 24 CC BY 2019 by Illustrative Mathematics 8

7. Sample response: means 10 yards of light fabric and 1 yard of dark fabric. This

is 11 yards of fabric altogether, which is more than 9.5. It costs $103 because

This is less than the $110 maximum.

Activity Synthesis

Focus the discussion on the last two questions. Select previously identified students to
share how they identified a pair of values that meet both constraints. Sequence their
presentation in the order listed in the Activity Narrative, which is from less systematic to
more systematic.

Select students who graphed both inequalities on the same plane to display their graphs
for all to see.

If no students did this on their own, display the embedded applet in the online materials
for all to see.

Desmos applet is available in the digital version of the materials.

Consider selecting first one inequality, then the other, then both simultaneously.

(The Desmos graphs show both variables being restricted to positive values, but students
are not expected to do this when graphing. Consider discussing with students why it
makes sense in this situation to disregard negative values.)

Discuss the meaning of a pair of values that satisfy both inequalities. Emphasize that, in
this situation, it refers to the amount of fabric of each color that meets both the length and
cost requirements.

Explain to students that the two inequalities representing the constraints in the same
situation form a system of linear inequalities. In a system of linear equations, the
solutions can be represented by one or more points where the graphs of the equations
intersect. The solutions to a system of inequalities are all points in the region where the
graphs of the two inequalities overlap, because those points represent all pairs of values
that make both inequalities true.

24.3 Remember These Situations?

10 minutes
In this activity, students write systems of inequalities that represent situations and find the
solutions by graphing. Students have encountered the same situations and constraints in a
previous lesson, so the main work here is on thinking about each pair of constraints as a

Algebra1 Unit 2 Lesson 24 CC BY 2019 by Illustrative Mathematics 9

system, finding the solution region of each inequality, and identifying a point in the region
where the graphs overlap as a solution to the system.

Addressing

• HSA-CED.A.3
• HSA-REI.D.12

Instructional Routines

• Graph It
• MLR8: Discussion Supports

Launch
Arrange students in groups of 2–4. Explain to students that they will now revisit some
situations they have seen and graphed in an earlier lesson. Assign one situation to each
group member (or allow them to choose one situation) and ask them to answer the
questions. Provide access to graphing technology.

Give students 5-6 minutes of quiet work time, and then time to discuss their responses
and graph with their group.

Support for English Language Learners

Speaking: MLR8 Discussion Supports. Use this routine to support small-group
discussion. At the appropriate time, give students 2–3 minutes to plan what they will
say when they present their responses and graph to their group. Encourage students
to consider what details are important to share and to think about how they will
explain their reasoning using mathematical language.
Design Principle(s): Support sense-making; Maximize meta-awareness

Support for Students with Disabilities

Action and Expression: Internalize Executive Functions. To support development of
organizational skills, check in with students within the first 2–3 minutes of work time.
Look for students who are organizing their information, understanding the need for
two equations, and strategizing as to how to overlay them onto a single graph. Supports
accessibility for: Memory; Organization

Algebra1 Unit 2 Lesson 24 CC BY 2019 by Illustrative Mathematics 10

Anticipated Misconceptions
Students may need reminding what “a solution to the system” would be in these specific
contexts. (First find a point where the total money in the two accounts totals less than or
equal to $600. Now make sure the money in the saving account is also at least $50.)

Student Task Statement
Here are some situations you have seen before. Answer the questions for
one situation.

Bank Accounts

• A customer opens a checking account and a savings account at a bank. They

will deposit a maximum of $600, some in the checking account and some in
the savings account. (They might not deposit all of it and keep some of the
money as cash.)

• The bank requires a minimum balance of $50 in the savings account. It does

not matter how much money is kept in the checking account.

Concert Tickets

• Two kinds of tickets to an outdoor concert were sold: lawn tickets and seat

tickets. Fewer than 400 tickets in total were sold.

• Lawn tickets cost $30 each and seat tickets cost $50 each. The organizers want

to make at least $14,000 from ticket sales.

Advertising Packages

• An advertising agency offers two packages for small businesses who need

advertising services. A basic package includes only design services. A premium
package includes design and promotion. The agency's goal is to sell at least 60
packages in total.

• The basic advertising package has a value of $1,000 and the premium package

has a value of $2,500. The goal of the agency is to sell more than $60,000
worth of small-business advertising packages.

Algebra1 Unit 2 Lesson 24 CC BY 2019 by Illustrative Mathematics 11

1. Write a system of inequalities to represent the
constraints. Specify what each variable represents.

2. Use technology to graph the inequalities and sketch
the solution regions. Include labels and scales for the
axes.

3. Identify a solution to the system. Explain what the numbers mean in the
situation.

Student Response
Bank Accounts

1. and , where represents amount
deposited in checking account and represents the
amount deposited in savings account.

2. See graph.

3. Sample response: , or $150 in the checking

account and $300 in the savings account. This

combination meets both requirements. The account

owner deposits more than $50 in the savings account

and no more than $600 all together.

Concert Tickets

Algebra1 Unit 2 Lesson 24 CC BY 2019 by Illustrative Mathematics 12

1. and , where

represents the number of lawn tickets and

represents the number of seat tickets.

2. See graph.

3. Sample response: or 100 lawn tickets and

250 seat tickets. This combination of lawn tickets and

seat tickets meet both constraints. The sum of tickets

is 350, so it is fewer than 400. The dollar amount

earned would be , which is

or 15,500. This is more that $14,000.

Advertising Packages

1. and , where

represents the number of basic packages and

represents the number of premium packages.

2. See graph.

3. Sample response: or 30 basic packages and 40

premium packages. This combination meets both the

constraints on the packages sold and the dollar

amount collected. , so the total number

of packages sold is more than 60. The amount

collected is , which is

or 130,000. This is more than

$60,000.

Activity Synthesis

Much of the discussion would have happened in small groups. During the whole-class
discussion, emphasize the meaning of a point in the region where two graphs of linear
inequalities overlap. Make sure students understand that all the points in that region
represent values that simultaneously meet both constraints in the situation.

If time permits, ask students: "Why does it make sense to think of the two inequalities in
each situation as a system and find the solutions to the system, instead of only to
individual inequalities?" (If both constraints in the situation must be met, then we need to
find values that satisfy both inequalities.)

Algebra1 Unit 2 Lesson 24 CC BY 2019 by Illustrative Mathematics 13