Hi Jon,
I am going to assume that you want the volume of dirt required in cubic yards. The volume expression is
length width average depth
but to obtain the volume in cubic yards you need to express the dimensions in yards not feet. There are 3 feet in a yard so
15 feet = ^{15}/_{3} = 5 yards and
36 feet = ^{36}/_{3} = 12 yards
If the depth slopes gradually from 3 feet to 8 feet then the average depth is
^{(3 + 8)}/_{2} = 5.5 feet. Thus the average depth in yards is
5.5 feet = ^{5.5}/_{3} = 1.83 yards
Hence the volume of dirt required is
5 12 1.83 = 110 cubic yards.
Penny
