Answer:

Each person’s charges can be determined by using $15 + $3n. The following

equations show the cost for each.

Mary—15 + 3(2) = 21

Tom—15 + 3(4) = 27

 

The cost for any number of hours (n) can be found by using $15 + 3n, where n is the number of hours talked.

 

 

Initial

Hour 1

Hour 2

Hour 3

Hour 4

Mary

$15

$18

$21

 

 

Tom

$15

$18

$21

$24

$27

                                                                                                                                   

 

Teacher Notes:

Students in grades 3–5 can model a variety of situations, including geometric patterns, real-world situations, and scientific experiments. Sometimes they will use their model to predict the next element in a pattern. At other times, they may make a general statement about how one variable is related to another variable: for example, if a sandwich costs $3, you can figure out how much any number of sandwiches costs by multiplying that number by 3. In modeling situations that involve real-world data, students need to know that their predictions may not always match the observed outcomes for a variety of different reasons. Students should also begin to understand that different models for the same situation can give the same results.11

 

11 National Council of Teachers of Mathematics. (2000). Principles and standards for school mathematics (pp. 162–163). Reston, VA: Author.