An inverted cone, (vertex down), is 20 inches tall and has a…

An inverted cone, (vertex down), is 20 inches tall and has a base radius of 8 inches.  The cone was initially full of water.  The water now drains from the cone at the constant rate of 15 cubic inches per second.  The water’s surface level falls as a result.  At what rate is the water level falling when the water is halfway down the cone? The volume of a cone of height h and radius r is