Algebra 1. I need a equestion that models total cost as a function of the number minutes spent washing the car?

A car wash charges a flat rate for the first 8 minutes of washing time and also charges a certain amount for each additional minute of washing time. Guns took 12 minutes to wash her car and it costs her a total of $5.00. Kris took 17 minutes to wash his truck at a total of$ 6.25.