Chassity,

Let's look for the correct expression first. I know that the volume of a cone with base radius r units and height h units is given by

V = 1/3 π r² h cubic units,

and I know that the circumference of a circle of radius r units is given by

C = 2 π r units.

Solving this last equation for r and substituting in the volume expression I get the volume as

V = (C² h)/(12 π) cubic units.

Since you want the volume in cubic yards I would convert the measurements to yards. There are 3 feet in a yard so 120 feet is 120/3 = 40 yards and 20 feet is 20/3 = 6.67 yards. Hence the volume of the gravel pile is approximately

V = (40² × 6.67)/(12 π) = 283 cubic yards.

Penny