A conical tank (with vertex down) is 12 feet across the top and 18 feet deep. If water is flowing into the tank at a rate of 18 cubic feet per minute, find the rate of change of the depth of the water when the water is 10 feet deep. 81/20π ft/min 9/100π ft/min 9/40π ft/min 81/50π ft/min 81/200π ft/min