Question

A long distance telephone company advertised the rates of 5 minutes for 85¢ and 10 minutes for $1.10.

Answer 1

Let x represent the number of minutes and y the cost. Then
y - 0.85 = (1.10 - 0.85)/(10 - 5) (x - 5)
y - 0.85 = 0.25/5 (x - 5) = 1/20 (x - 5) = 1/20 x - 1/4
y = 0.05x - 0.25 + 0.85 = 0.05x + 0.6
y = 0.05x + 0.6

The cost of 25 minutes = 0.05(25) + 0.6 = 1.25 + 0.6 = $1.85