Question

The monthly cost (in dollars) of a long-distance phone plan is a linear function of the total calling time (in minutes). The monthly cost for 52 minutes of calls is $14.06 and the monthly cost for 78 minutes is $17.18. What is the monthly cost for 73 minutes of calls?

Answer 1

16.58

Explanation:

as you can see from the graph, there's a monthly fixed cost on Y axis. so you need to figure out 1: how much per minute. and 2. the monthly fixed fee. you've got 2 scenarios = 2 equations, for 2 variables. so you can solve both.

assume: x=per min rate; m = monthly fixed cost.

1. 14.06 =52x+m

2. 17.18 =78x+m

use equation 2 minus equation 1 & you get

3.12 = 26 x. so x = 0.12

plug this into equation 1: 14.06 =52*0.12 +m so m=7.82

now for 73 mins the cost = 73*0.12 +7.82 =16.58