A barber offers two options at his barbershop; a $15 regular haircut and a $20 deluxe haircut. On a certain day the barber gave r regular haircuts and 3 fewer deluxe haircuts than regular haircuts. He earned $500 total from the two types of haircuts. Which of the following equations best models this situation?
a) 15 + 20(r - 3) = 500
b) 15 + 20(r + 3) = 500
c) 15(r - 3) + 20r = 500
d) 15(r + 3) + 20r = 500