Phillip,

I am going to find the distance in yards and then multiply by 3 to convert it to feet. The width of 12 feet is 12/3 = 4 yards and the depth of 4 inches is 4/36 = 1/9 yards. If the length is L yards then the volume of dirt is

volume = length × width × depth = L × 4 × 1/9 cubic yards.

You have 15 cubic yards and hence

15 = L × 4 × 1/9 = L × 4/9 cubic yards.

Multiplying each side by 9/4 gives

15 × 9/4 = L yards

or

L = 33.75 yards.

Thus L = 3 × 33.75 = 101.25 feet.

Penny