Gravel is being dumped from a conveyor belt at a rate of 40 cubic: feet per minute. It forms a pile in the shape of a right circular cone whose base diameter and height are always equal to each other (both are growing larger as more gravel is dumped). How fast is the height of the pile increasing when the pile is 21 feet high? Recall that the volume of a right circular oone with height h and radius of the base r is given by V = m'2h