A truck is rented at $40 per day plus a charge per mile of use. The truck traveled 15 miles in one day, and the total charge was $115. Which equation can be used to find the charge x, in dollars, per mile traveled?

a.40x + 15 = 115

b. 40 + 15x = 115

c.40x + 15x = 115

d.40 + 15 = 115x