Lesson 3

The purpose of this warm-up is to encourage students to recognize that in some situations it is helpful to think about a multiplicative increase rather than an additive increase. These situations can be described in terms of percent increase (situations in which an increase is obtained by adding a certain percentage of a quantity to itself). In this situation, total points for each sports team increase from Game 1 to Game 2, however the increases are different percentages of the first scores. For example, an increase of 8 points from 100 to 108 is not as significant as an increase from 4 points to 12 points, because in the first increase is only 8% of the original value while the second is 200% of the original value.

As students share things they notice, listen for language students use to discuss the significance of the different increases. For example, students may say that the baseball team tripled their score, which would be like the basketball team going from 100 to 300 points in the next game.

Arrange students in groups of 2. Tell students they will look at a table and think of at least one thing they notice and at least one thing they wonder. Display the table for all to see and give 1 minute of quiet think time. Ask students to give a signal when they have noticed something about the teams’ scores. Invite students to share their ideas; record and display their responses for all to see. If no students wonder which team improved the most, direct them to the second question and give them 1 minute to work with a partner.

Students may say that the football team improved the least because the 8 points could have been scored from only 1 touchdown in football, but it would have to be 3 or 4 baskets in basketball and 8 separate runs in baseball. Prompt students to look at the significance of the 8 additional points in the context of each team’s score in the game 1, rather than the mechanics of scoring in each sport.

Poll students on which team they think improved the most. Ask a student who thinks they all improved by the same amount to share their reasoning (each team increased its score by 8 points). Then, ask a few students who said the baseball team improved the most to share their reasoning. There is no need to invoke the phrase “percent increase” or to express the change as a percent of the game 1 score at this time, but you want to plant the idea that it sometimes makes sense to describe a change relative to a starting amount, instead of just looking at absolute change. In the course of discussion, though, it may be natural to say things like the basketball team improved their score by 8% of their game 1 score, the football team improved by nearly \(\frac13\) of their game 1 score, while the baseball team tripled their game 1 score.

In this activity, students are given a percent increase and use it to calculate the new value, rather than being given the original and new values to calculate the percent increase. Students can solve the problems using the double number lines but the discussion that follows will be the connection to what the words specifically mean (MP6).

As students work, monitor for students who complete the double number line diagram and any student that uses other methods to correctly reason about the problem.

Arrange students in groups of 2. Give students 5 minutes of quiet work time followed by partner then whole-class discussion.

Have students use the double number line diagram if they need help figuring out 20% more.

Select students to share their reasoning for each problem. Start with a student that used the double number line diagram to solve the problem. Then, have students share other methods they used to solve the problem, such as multiplying by 1.2 and 0.8.

Ask students:

• Did the number of ounces of cereal in the cereal box increase or decrease?
• What percentage of the original amount of cereal is the new amount of cereal?
• Did the price of the shirt increase or decrease?
• What percentage of the original price of the shirt is the new price of the shirt?

Tell students that the change on the cereal box is an example of a percent increase. The discount on the shirt is an example of a percent decrease. You might want to mention that in both cases, 100% always corresponds to the original amount before the change; however, future activities will address this concept in depth.

The purpose of this activity is for students to understand that a percent increase of, say, 15% corresponds to 115% of the original amount, and a percent decrease of, say, 30% corresponds to 70% of the original amount.

Show students this image. 

Say, "Explain how each of these is related to the diagram."

1. \(x + \frac14 x\)
2. \(y=1.25x\)
3. 125%
4. An increase of 25%

1 minute of quiet think time followed by partner and then whole group discussion. 

If the amount of fruit increases by 40%, what percent of the original amount do you have?

If the amount of fruit decreases by 40%, what percent of the original amount do you have?

The purpose of this activity is to for students to evaluate claims about percentages within contexts in which common misunderstandings occur (MP3). The first question prompts students to think about the original pay of each employee. Since we do not know the pay for each employee, a higher pay raise percentage does not necessarily mean a higher dollar amount. Students could disagree based on the reasoning that Employee A makes $10 per hour, so a 50% raise would be an increase of $5 per hour. However, if Employee B makes $20 per hour, a 45% raise would be an increase of $9 per hour. On the other hand, students could agree with this statement if they think both employees make the same amount. The second question prompts students to reason about the effect of trying to combine percentages. Ask students to discuss the following with their partner:

• Did you agree or disagree with one another?
• How did you test out your ideas?
• What did you notice to be true after testing out some numbers in the statement?

Arrange students in groups of 2. Give students 1 minute of quiet work time followed by partner and whole-class discussions. Refer to MLR 3 (Clarify, Critique, Correct) for prompts to build student language for evaluating a statement. One example is to use the "Always-Sometimes-Never" approach for helping students determine the validity of the statements.

Students may agree with both statements at first as the statements themselves are common misconceptions. Ask these students to assign values to the pieces of the statement and test them out. For example, assign how much each employee makes in the first statement or how much each shirt costs in the second statement.

Poll students if they agree or disagree with each statement and ask students to explain their reasoning. Record and display student explanations for all to see. To involve more students in the conversation, consider asking some of the following questions:

• “Do you agree or disagree? Why?”
• “Did anyone think about the statement in the same way but would explain it differently?”
• “Does anyone want to add on to _______’s reasoning?”