A car wash charges a flat fee for the first 8 minutes of washing time and also charges a certain amount of each additional minute of washing time. Gina took 12 minutes to wash her car and it costs her total of $5. Kris took 17 minutes to wash his truck at a total cost of $6.25. Write an equation that models the total cost of a car wash as a function of the number of minutes spent washing a vehicle.