Lesson 3
Representing Proportional Relationships
3.1: Number Talk: Multiplication (5 minutes)
Warmup
This Number Talk encourages students to think about the numbers in a computation problem and rely on what they know about structure, patterns, multiplication, fractions, and decimals to mentally solve a problem. Only one problem is presented to allow students to share a variety of strategies for multiplication. Notice how students handle the multiplication by a decimal. Some students may use fraction equivalencies while others may use the decimal given in the problem. For each of those choices, ask students why they made that decision.
Launch
Display one problem at a time. Give students 1 minute of quiet think time per problem and ask them to give a signal when they have an answer and a strategy. Follow with a wholeclass discussion. Be sure to elicit as many strategies as possible.
Supports accessibility for: Memory; Organization
Student Facing
Find the value of each product mentally.
\(15 \boldcdot 2\)
\(15 \boldcdot 0.5\)
\(15 \boldcdot 0.25\)
\(15 \boldcdot (2.25)\)
Student Response
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Activity Synthesis
Invite students to share their strategies. Use MLR 2 (Collect and Display) to record and display student explanations for all to see. Ask students to explain their choice of either using a decimal or fraction for \(2.25\) in their solution path. To involve more students in the conversation, consider asking:
 “Who can restate _____’s reasoning in a different way?”
 “Did anyone solve the problem the same way but would explain it differently?”
 “Did anyone solve the problem in a different way?”
 “Does anyone want to add on to _____’s strategy?”
 “Do you agree or disagree? Why?”
At the end of discussion, if time permits, ask a few students to share a story problem or context that \(15\boldcdot (2.25)\) would represent.
Design Principle(s): Optimize output (for explanation)
3.2: Representations of Proportional Relationships (15 minutes)
Activity
The purpose of this activity is for students to graph a proportional relationship when given a blank pair of axes. They will need to label and scale the axes appropriately before adding the line representing the given relationship. In each problem, students are given two representations and asked to create two more representations so that each relationship has a description, graph, table, and equation. Then, they explain how they know these are different representations of the same situation (MP3). In the next lesson, students will use these skills to compare two proportional relationships represented in different ways.
Identify students making particularly clear graphs and using situationappropriate scales for their axes. For example, since the second problem is about a car wash, the scale for the axis showing the number of cars does not need to extend into the thousands.
Launch
Arrange students in groups of 2. Provide access to straightedges. Give 3 minutes of quiet work time for students to begin the first problem and then tell students to check in with their partners to compare tables and how they are labeling and scaling the axes. Ask students to pause their work and select a few students to share what scale they are using for the axes and why they chose it. It is important to note that the scale chosen should be reasonable based on the context. For example, using a very small scale for steps taken does not make sense.
Give partners time to finish the remaining problems and follow with a wholeclass discussion.
Supports accessibility for: Language; Organization
Student Facing

Here are two ways to represent a situation.
Description:
Jada and Noah counted the number of steps they took to walk a set distance. To walk the same distance, Jada took 8 steps while Noah took 10 steps. Then they found that when Noah took 15 steps, Jada took 12 steps.
Equation:
Let \(x\) represent the number of steps Jada takes and let \(y\) represent the number of steps Noah takes. \(\displaystyle y=\frac54x\)

Create a table that represents this situation with at least 3 pairs of values.

Graph this relationship and label the axes.

How can you see or calculate the constant of proportionality in each representation? What does it mean?

Explain how you can tell that the equation, description, graph, and table all represent the same situation.


Here are two ways to represent a situation.
Description:
The Origami Club is doing a car wash fundraiser to raise money for a trip. They charge the same price for every car. After 11 cars, they raised a total of $93.50. After 23 cars, they raised a total of $195.50.
Table:
number of
carsamount raised
in dollars11 93.50 23 195.50  Write an equation that represents this situation. (Use \(c\) to represent number of cars and use \(m\) to represent amount raised in dollars.)
 Create a graph that represents this situation.
 How can you see or calculate the constant of proportionality in each representation? What does it mean?
 Explain how you can tell that the equation, description, graph, and table all represent the same situation.
Student Response
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Activity Synthesis
Ask previously identified students to share their graphs and how they chose the scales for their axes. If possible, display several graphs from each question for all to see as students share.
Ask students “Which representation makes it more difficult (and less difficult) to calculate the constant of proportionality? Why?” and give 1 minute of quiet think time. Invite several students to share their responses.
Tell students that the constant of proportionality can be thought of as the rate of change of one variable with respect to the other. In the case of Jada and Noah, the rate of change of \(y\), the number of steps Noah takes, with respect to \(x\), the number of steps Jada takes, is \(\frac54\) Noah steps per Jada steps. In the case of the Origami Club’s car wash, the rate of change of \(m\), the amount they raise in dollars, with respect to \(c\), the number of cars they clean, is 8.50 dollars per car.
Design Principle(s): Optimize output; Cultivate conversation
3.3: Info Gap: Proportional Relationships (15 minutes)
Activity
This info gap activity gives students an opportunity to determine and request the information needed when working with proportional relationships. In order to graph the relationship and the requested information, students need to think carefully about how they can scale the axes.
The info gap structure requires students to make sense of problems by determining what information is necessary, and then asking for information they need to solve it. This may take several rounds of discussion if their first requests do not yield the information they need (MP1). It also allows them to refine the language they use and ask increasingly more precise questions until they get the information they need (MP6).
Here is the text of one of the cards for reference and planning:
Listen for how students request (and supply) information about the relationship between the two ingredients. Identify students using different scales for their graphs that show clearly the requested information to share during the discussion.
Launch
Tell students that they will continue their work graphing proportional relationships. Explain the Info Gap and consider demonstrating the protocol if students are unfamiliar with it. Arrange students in groups of 2. Provide access to straightedges. In each group, distribute a problem card to one student and a data card to the other student. After you review their work on the first problem, give them the cards for the second problem and instruct them to switch roles.
Supports accessibility for: Language; Memory
Design Principle(s): Cultivate Conversation
Student Facing
Your teacher will give you either a problem card or a data card. Do not show or read your card to your partner.
If your teacher gives you the problem card:

Silently read your card and think about what information you need to be able to answer the question.

Ask your partner for the specific information that you need.

Explain how you are using the information to solve the problem.
Continue to ask questions until you have enough information to solve the problem.

Share the problem card and solve the problem independently.

Read the data card and discuss your reasoning.
If your teacher gives you the data card:

Silently read your card.

Ask your partner “What specific information do you need?” and wait for them to ask for information.
If your partner asks for information that is not on the card, do not do the calculations for them. Tell them you don’t have that information.

Before sharing the information, ask “Why do you need that information?” Listen to your partner’s reasoning and ask clarifying questions.

Read the problem card and solve the problem independently.

Share the data card and discuss your reasoning.
Pause here so your teacher can review your work. Ask your teacher for a new set of cards and repeat the activity, trading roles with your partner.
Student Response
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Student Facing
Are you ready for more?
Ten people can dig five holes in three hours. If \(n\) people digging at the same rate dig \(m\) holes in \(d\) hours:
 Is \(n\) proportional to \(m\) when \(d=3\)?
 Is \(n\) proportional to \(d\) when \(m=5\)?
 Is \(m\) proportional to \(d\) when \(n=10\)?
Student Response
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Anticipated Misconceptions
Some students may be unsure how large to make their scale before they answer the question on the card. Encourage these students to answer the question on their card and then think about how to scale their graph.
Activity Synthesis
After students have completed their work, ask previously identified students to share their graphs and explain how they chose their axis. Some guiding questions:
 “Other than the answer, what information would have been nice to have?”
 “How did you decide what to label the two axes?”
 “How did you decide to scale the horizontal axis? The vertical?”
 “What was the rate of change of grams of honey per cups of flour? Where can you see this on the graph you made?” (4.5 grams of honey per cup of flour.)
 “What was the rate of change of grams of salt per cups of flour? Where can you see this on the graph you made?” (2.5 grams of salt per cups of flour)
Lesson Synthesis
Lesson Synthesis
Consider asking some of the following questions.
 “The proportional relationship \(y=5.5x\) includes the point \((18, 99)\) on its graph. How could you choose a scale for a pair of axes with a 10 by 10 grid to show this point?” (Have each grid line represent 10 or 20 units.)
 “What are some things you learned about graphing today that you are going to try to remember for later?”
3.4: Cooldown  Graph the Relationship (5 minutes)
CoolDown
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Student Lesson Summary
Student Facing
Proportional relationships can be represented in multiple ways. Which representation we choose depends on the purpose. And when we create representations we can choose helpful values by paying attention to the context. For example, a stew recipe calls for 3 carrots for every 2 potatoes. One way to represent this is using an equation. If there are \(p\) potatoes and \(c\) carrots, then \(c = \frac32p\).
Suppose we want to make a large batch of this recipe for a family gathering, using 150 potatoes. To find the number of carrots we could just use the equation: \(\frac32\boldcdot 150= 225\) carrots.
Now suppose the recipe is used in a restaurant that makes the stew in large batches of different sizes depending on how busy a day it is, using up to 300 potatoes at at time.
Then we might make a graph to show how many carrots are needed for different amounts of potatoes. We set up a pair of coordinate axes with a scale from 0 to 300 along the horizontal axis and 0 to 450 on the vertical axis, because \(450 = \frac32\boldcdot 300\). Then we can read how many carrots are needed for any number of potatoes up to 300.
Or if the recipe is used in a food factory that produces very large quantities and the potatoes come in bags of 150, we might just make a table of values showing the number of carrots needed for different multiplies of 150.
number of potatoes  number of carrots 

150  225 
300  450 
450  675 
600  900 
No matter the representation or the scale used, the constant of proportionality, \(\frac32\), is evident in each. In the equation it is the number we multiply \(p\) by; in the graph, it is the slope; and in the table, it is the number we multiply values in the left column to get numbers in the right column. We can think of the constant of proportionality as a rate of change of \(c\) with respect to \(p\). In this case the rate of change is \(\frac32\) carrots per potato.