application problem with a linear function

Question

Suppose that the credit remaining on a phone card (in dollars) is a linear function of the total calling time (in minutes ). When graphed, the function gives a line with a slope of -0.15. There is $34.50 in credit remaining on the card after 30 minutes of calls. How much credit will there be after 45 minutes of calls?

Answer 1

C = -0.15M + b

34.50 = -0.15(30) + b

34.5 = -4.5 + b

b = 34.5+4.5 = 39

C = 39 - 0.15M

C = 39 -0.15(45) = 39 - 6.75 = $32.25 Credit left after 45 minutes