standard_description: Recognize and represent proportional relationships between quantities.
problem_activity: Activity
The purpose of this activity is for students to use double number line diagrams to represent situations of percent increase and decrease. Additionally, students identify the original and new amount in the double number lines to reinforce that the original amount pertains to 100%.
As students work on the task, monitor for students who created various equations for the last question.
Launch
Arrange students in groups of 2. Give 5–8 minutes of quiet work time. After 5 minutes allow students to work with a partner or to continue to work alone.
Action and Expression: Internalize Executive Functions.
Chunk this task into more manageable parts to support students who benefit from support with organization and problem solving. For example, present one question at a time and monitor students to ensure they are making progress throughout the activity.
Supports accessibility for: Organization; Attention
Student Facing
For each problem, complete the double number line diagram to show the percentages that correspond to the original amount and to the new amount.
The gas tank in dad’s car holds 12 gallons. The gas tank in mom’s truck holds 50% more than that. How much gas does the truck’s tank hold?
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At a movie theater, the size of popcorn bags decreased 20%. If the old bags held 15 cups of popcorn, how much do the new bags hold?
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A school had 1,200 students last year and only 1,080 students this year. What was the percentage decrease in the number of students?
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One week gas was \$1.25 per gallon. The next week gas was \$1.50 per gallon. By what percentage did the price increase?
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After a 25% discount, the price of a T-shirt was \$12. What was the price before the discount?
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Compared to last year, the population of Boom Town has increased 25%. The population is now 6,600. What was the population last year?
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Student Response
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Anticipated Misconceptions
Students may continue to struggle to recognize the original amount and new amount with the proper percentages on the double number line.  Remind them that the original amount always corresponds to 100%.
Activity Synthesis
Select students to share the values they identified as original amount and the new amount for a few problems. Discuss how 100% always corresponds to the original value and when there is an increase in the value the new value corresponds to a percentage greater than the original 100%.
Select students to share the different equations they came up with. Discuss how the distributive property is useful for finding the percentage that corresponds with the new value instead of the percentage of the change.
Discuss how solving problems about percent change may require either multiplying or dividing numbers. It can be confusing, but it helps to first express the relationship as an equation and then think about how you can find the unknown number. Looking at the examples below, the first two require multiplication, but the others require division.
Using the structure A% of B is C:
\((1.5) \boldcdot 12 = c\)
\((0.80) \boldcdot 15 = c\)
\(a \boldcdot (1,\!200) = 1,\!080\)
\(a \boldcdot (1.25) = 1.50\)
\((0.75) \boldcdot b = 12\)
\((1.25) \boldcdot b = 6,\!600\)
Representing, Speaking: MLR7 Compare and Connect.
Use this routine after selected students share the values they identified for the original amount and the new amount for a few problems. Ask students what is the same and what is different about the ways double number lines were used to represent percent increase and decrease for the different situations. Call students' attention to how the original amount is represented as 100% in the tape diagrams and in some of the equations. This will help strengthen students' mathematical language use and reasoning based on percent increase and decrease.
Design Principle(s): Maximize meta-awareness


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