The cost of a parking permit consists of a one-time administration fee plus a monthly fee. a permit purchased for 12 months costs $660. a permit purchased for 15 months costs $810. what is the administration fee?

The general equation that may be used to determine the total cost of the parking permit,
                                       P = A + mt
where P is the total cost, A is the administration fee, m is the monthly fee and t is the number of months. Substituting the known values to the equation,   
                  (1)       660 = A + 12m
                  (2)       810 = A + 15 m

Solving for the solutions of the equation gives A = 60 and m = 50. Thus, the administration fee is $60.

What's the denominator