Mathematics Big Idea and Major Topic: Interpreting and Writing Linear Equations
More specific statement of the topic: Examine linear functions, create a table of values, graph linear functions, and make predictions.
What the students already know: Ability to graph a point, create a table of values, and evaluate expressions.
What part will the students have to figure out on their own? The relationship between two variables, writing linear equations.
Context: Explain that cell phone plans have both an initial cost and a price per minute for talking. Therefore, there is a linear relationship between the number of minutes talked and the total cost.
Statement of the task: Given a cell phone plan, students calculate cell phone charges based on minutes talking, graph the values, make predictions based on the graph, and create a linear equation using the data.
Where might they get stuck? Calculating and plotting points correctly. Finding relationships in the data.
What questions can I use to prompt them without giving away answers?
What is the problem asking you to do? What do you notice about all the points?
How does your equation represent the data? Is your equation accurate? Does your team agree with the results?
Improving Student Achievement in Algebra, Grades 7-12. The DuPage1 Mathematics
Network. DuPage Regional Office of Education. May 2007
1

LINEAR EQUATIONS
Cell phones are a constant in modern life. Varying rates exist in different cell phone plans. Use the SPREE plan to explore a linear relationship.
Cell Phone Plan
SPREE: $10.00 plus $0.16 each additional minute
1. Consider the following questions: a. Jack talks for 20 minutes. How much is his bill? b. Jack talks for 40 minutes. How much is his bill? c. Jack talks for 50 minutes. How much is his bill?
2. Complete a table of values
based on cell phone usage:
3. Plot the data points from the
table of values. Connect the
Minutes
10
15
20
25
30
points using a straight edge.
$ amount
35
40
45
50
55
60
Improving Student Achievement in Algebra, Grades 7-12. The DuPage1 Mathematics
Network. DuPage Regional Office of Education. May 2007
2

4. Use the graph to estimate the following: a. What is the cost of talking 0 minutes? b. What is the cost of talking 42 minutes? c. How many minutes can you talk for $17?
5. a. Write an expression for the cost of talking x minutes? b. Create a linear equation for the cell phone plan where x is the
number of minutes and y represents the total cost.
6. Using the equation you created, determine the cost for the following minutes
of talk time. a. 100 minutes b. 200 minutes c. 1000 minutes
Improving Student Achievement in Algebra, Grades 7-12. The DuPage1 Mathematics
Network. DuPage Regional Office of Education. May 2007
3