*C = πd*). After 72 revolutions, the larger wheel will have gone a distance of 72(18π) inches. The circumference of the smaller wheel is 12π inches, and if we call the number of revolutions that the smaller wheel makes *r*, then we know that 12π*r* = 72(18π). Dividing both sides by 12π gives us *r* = 6(18) or 108 revolutions. Note that in this problem we have used the relation *distance = rate × time*, where the time for both wheels is a fixed quantity.