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 40 minutes of calls is $16.03 and the monthly cost for 81 minutes is $19.72. What is the monthly cost for 64 minutes of calls?​

Answer 1

18.19

Explanation:

$ = linear f(t)

$16.03 = f(40) = fixed fee + per min fee = x + 40y

$19.72 = f(81) = fixed fee + per min fee = x + 81 y

use above 2 equations to minus the other

you get:

19.72-16.03 = 41y

so y = 0.09

so x = 16.03 - 40*0.09 =12.43

so for 64 mins

cost = 12.43 + 0.09*64 =18.19