Skip to main content Skip to main content

Insights Into Algebra 1: Teaching for Learning

Linear Functions and Inequalities Lesson Plan 1: The Phone Bill Problem – Linear Functions

In this lesson, students will learn how to write an equation of a linear function when given a set of data. They will interpret the meaning of the slope and y-intercept and then use the equation to find other values of x and y.

Time Allotment:
One 50-minute class period

Subject Matter:
Linear functions

Learning Objectives:
Students will be able to:

  • Make sense of a set of data and plot it on a graph.
  • Find the equation of the line that contains the data points.
  • Understand the meaning of the slope and y-intercept.
  • Use the equation to predict other x- and y-values.

Principles and Standards for School Mathematics, National Council of Teachers of Mathematics (NCTM), 2000:

NCTM Algebra Standard for Grades 6-8

NCTM Algebra Standard for Grades 9-12

Teacher Supplies


Students will need the following:

Teachers Activities and Assignment


Introductory Activity:

1. Explain to the class that the handout contains a copy of an actual phone bill. Have students look at the phone bill data – code, minutes, and cost – and determine whether to include all of the calls in the data set.

2. Students should notice that all of the calls have the code “EC” except for the second call, which is coded “NC.” They might hypothesize that they should eliminate the call coded NC from the data set.

3. Prompt students to examine the cost and length of the first two calls.

4. Students should notice that the EC call is more expensive than the NC call, even though it’s shorter. They should conclude that the class should eliminate the call coded NC from the data set. (You may want to tell them that NC is a lower billing rate applied to nighttime calls.)

Learning Activities:

1. Ask the students to arrange the EC calls in a table, from the shortest in length to the longest.

2. Make a scatterplot of the data using a graphing calculator. Ask students to discuss the meaning of the x- and y-values and to decide the appropriate values to use in the window setting. In the video, the students proposed the following: x = minutes, y = cost of the call, xmin = 0, xmax = 50, xscl = 10, ymin = 0, ymax = 20, yscl = 5. Make sure students can justify their choices, and remind them that these values represent the domain and range in this problem.

3. Elicit from the class that if the data points were connected, they would form a line.

4. Allow students time to devise a plan for finding the equation of the line formed by connecting the data points.

5. Ask students to report on their plan. Students should mention that they would find the slope and y-intercept of the line. Focus on the slope first and make sure students understand how to use the data to find the slope.

6. Give the students time to choose two data points and find the slope (m = 0.24).

7. Ask students to discuss how to find the value of the y-intercept. They should conclude that the y-intercept, b, is 0.85.

8. Have the class verify the values for both the slope and the y-intercept and determine that the equation of the line is y = 0.24x + 0.85.

9. Check the equation by graphing it and decide how well the equation fits the data. The students should notice that the equation falls on all of the data points, and is therefore a perfect fit.

10. Ask students to consider whether or not it matters which two data points they choose to find the equation of the line. Students should determine that because the data points all lie on the line, they could choose any two and arrive at the same equation.

11. Trace to x = 8 and y = 2.77 and ask students to discuss the meaning of this point. They should understand that this point means that if you talk for eight minutes, the cost of the call would be $2.77.

12. Ask students to choose another x-value and find the corresponding y-value, and explain how they found it. Students may choose to find the value by using a table or a graph, or by substituting into the equation.

13. Ask students to look at the table values for x = 0 to x = 5 and describe any patterns they see. Students should notice that for each increase of x, the y-values increase by $0.24.

14. Give the students some time to determine the meaning of the slope and the y-intercept in this problem context.

15. Make sure they understand that the slope represents the fact that the increase in cost for the call is 24 cents per minute and the y-intercept of 0.85 represents the initial fee for the call.

16. Ask the class to determine the cost of a one-minute phone call. Students should report that this call would cost $1.09 ($0.85 as the initial fee plus $0.24 for up to one minute). Ask students how much it would cost if the call lasted only 15 seconds. Students should realize that any call of one minute or less would cost $1.09.

17. Write the equations using different variables and using function notation:

y = 0.24x + 0.85
C = 0.24t + 0.85
C(t) = 0.24t + 0.85

Explain to the class that these are different ways to express the same equation. To demonstrate that fact, ask students to substitute the same values into each equation.

18. Ask the class to find the cost of a one-hour phone call. Discuss the method they chose to solve the problem – table, graph, or equation – and their reason for choosing that method.

19. Ask students how much it would cost to talk on the phone for 24 hours. Students will first need to determine the number of minutes in 24 hours. For example:

20. Students should then substitute the number of minutes, 1440, into one of the equations above to find the cost. (Answer: $346.45)

21. Ask the students to determine how long a call lasted if it cost $6.13. (Answer: 22 minutes)

Culminating Activity/Assessment:

1. Ask students to write a paragraph that describes what they learned today, what they re-learned today, and what they liked about today’s lesson.

2. Assign practice problems for homework.

Related Standardized Test Questions

The questions below dealing with linear functions have been selected from various state and national assessments. Although the lesson above may not fully equip students to answer all such test questions successfully, students who participate in active lessons like this one will eventually develop the conceptual understanding needed to succeed on these and other state assessment questions.

  • Taken from the North Carolina End-of-Course Test, Algebra 1 (October 2003):

    The cost of a large pizza is given by the formula C(t) = 1.5t + 7.5, where C(t) is the cost of the pizza and t is the number of toppings. What does the slope represent?

    A. Number of toppings
    B. Cost per slice
    C. Cost of each topping (correct answer)
    D. Cost of pizza with no toppings

  • Taken from the North Carolina End-of-Course Test, Algebra 1 (October 2003):

    Denisha bought a car for $15,000. The value depreciates linearly. After three years, the value is $11,250. What is the amount of yearly depreciation?

    A. $2,000
    B. $1,500
    C. $1,250 (correct answer)
    D. $750

  • Taken from the Virginia Standards of Learning, Algebra 1 (2002):
    The graph of is shown below. If the line in the graph is shifted up 2 units, what is the equation of the new line?




    D. (correct answer)

  • Taken from the North Carolina End-of-Course Test, Algebra 1 (October 2003):Which of the following equations describes the data in the table below?

A. 2x + y = -27
B. x – y = -3
C. x + y = -21
D. 2x – y = 3 (correct answer)

Student Work: The Phone Bill Problem

Teacher Commentary:

Matthew took good notes. He had a difficult time with the spelling of the word “intercept,” spelling it three different ways in a matter of minutes. He also didn’t realize at first that m is for slope and b is for the y-intercept.

It was not always clear to me how he decided upon the answers to some of the questions we asked. He would just write “graph” or “table”.

I do see evidence that he understands what he is doing. I also see some unsure areas. I appreciated Matt’s comments at the end of the assignment and how he referred to this problem as a “day-to-day” problem.

I would not change this [assignment]. I have been using it for several years and it has served my students well at all levels of mathematical maturity. I just need to adapt the class to the [assignment].

Series Directory

Insights Into Algebra 1: Teaching for Learning


Produced by Thirteen/WNET. 2004.
  • ISBN: 1-57680-740-1