Question

Telephone company A charges $3.00 for the first minute of any long distance call and $0.50 for each additional minute. Telephone company B charges $2.00 for the first minute of any long distance call and $0.70 for each additional minute. If the cost of a call lasting x minutes, where x is a positive integer, is $15.00 more with telephone company B than with telephone company A, then what is the value of x?
costA= 3.00 + .5(x-1)
costB = 2.00 + .7(x-1)

2 + .7(x-1) - 3 - .5(x-1) = 15
.7x - .5x - .7 + .5 -1 = 15
.2x = 16.2
x = 81

check:
if x = 81
costA = 3+.5(80) = 43
costB = 2 + .7(80) = 58 , which is $15 more
what is the reason for multiplying the additional minute cost by (x-1) why not just X?

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