A basic cellphone plan costs 20 per month for 60 calling min

A basic cellphone plan costs $20 per month for 60 calling minutes. Additional time costs $0.40 per minute.

The formula C = 20 + 0.40(x - 60) gives the monthly cost for this plan, C, for x calling minutes, where x>60. How many calling minutes are possible for a monthly cost of at least $28 and at most $40?

Solution

We have C = 20 + 0.40(x - 60). When C = $ 28, we have 28 = 20 + 0.40 ( x - 60) or, 8 = 0.40( x -60) or, x - 60 = 8 / ( 0.4) or x - 60 = ( 8*100) /40 = 20. Therefore, x = 20 + 60 = 80 minutes. Thus, for $ 28, the number of possible calling minutes is 80.

Similarly, when C = $ 40, we have 40 = 20 + 0.40 ( x - 60) or, 20 = 0.40(x - 60) or x -60 = 20 / ( 0.4) = (20*100)/40 = 50. Therefore x = 50 + 60 = 110 minutes. Thus for $ 40, the number of possible calling minutes is 110..