# Lesson 18

Modeling Price Information

These materials, when encountered before Algebra 1, Unit 4, Lesson 18 support success in that lesson.

## 18.1: What’ll It Be? (10 minutes)

### Warm-up

This warm-up helps students get oriented to the situation they will be working with throughout this lesson. Students also make initial predictions based on very limited information.

### Student Facing

The points on the graph represent the average resale price of a toy in dollars as a function of time.

1. Use the information to predict the average resale price of the toy on day 12. Explain your reasoning.

### Activity Synthesis

The purpose of the discussion is to recognize that it can be difficult to make predictions based on very limited data and information.

Poll the class on their predictions. Ask selected students to explain why they made that prediction. Record and display the responses for all to see. If possible, display and reference the graph as students explain their reasoning.

Ask students, “What other information would help improve your predictions?” (More data or a more detailed explanation of what the toy is and what is happening would help me make a better prediction.)

## 18.2: Collectable Toy Price (15 minutes)

### Activity

In this activity students use some data to find an average rate of change and write a linear function to model data for the price of a collectable toy over several days. In the associated Algebra 1 lesson students examine battery life on a phone by modeling a graph with a function. Students are supported by this activity by being given some additional steps to think through while modeling a situation.

If possible, do not allow students to look at the next activity while they work on this activity. The next activity gives students additional information about the price that may affect their thinking for the questions here.

### Student Facing

The graph shows the average resale price for a toy in dollars as a function of time in days.

1. Estimate the average rate of change for the first 10 days.
2. Estimate the rate of change for the last 2 days shown.
3. Write a linear function, $$f$$, that models the data.
4. Predict the price of the toy after 12 days.

### Activity Synthesis

The purpose of the discussion is to provide insight into how to model data with functions. Select students to share their predictions, estimates, and model functions. After each response is shared, ask if there are other possible solutions from other students. Ask students,

• “How did you come up with your model function?” (I drew a line that went through the middle of the data and it was close to going through the points $$(0,5)$$ and $$(10,25)$$, so I wrote the equation of the line going through those points.)
• “On day 0 the price of the toy was \$5, does that mean your linear function should also have 5 as the vertical intercept?” (Not necessarily. It did work out in this case, but there could be another vertical intercept, especially if the point is very different from the rest of the data near zero.)
• “How did you use the average rate of change in your model function?” (I used the average rate of change for the 10 days as the slope of my function because it seemed to be a good fit.)
• “How confident are you in your prediction for the price of the toy after 12 days?” (I’m not very confident. While there does seem to be a nice upward trend from the data we have, at any point the price could drop significantly.)

### Activity

In this activity students get additional information about the average price of the toy which does not follow the same trend as in the previous activity. The additional information shows students that trends can change and that knowing additional data is helpful, but also knowing the situation can help make better predictions.

### Launch

After a few initial questions, students are asked to pause to get additional information from the teacher. During the pause, tell students that on day 13 the company that makes the toy released another shipment of the toy.

### Student Facing

After a few more days, a graph of the average price of the toy looks like this.

1. Draw a function (it does not need to be linear) that could model the data.
2. Use your graph to predict the average price of the toy after 12 days. How confident are you in this answer?
3. Pause here to get additional information from your teacher about the price of the toy. Based on the new information, do you have a new prediction for what happens to the average price of the toy after 12 days? Explain your reasoning.