7. An open hemispherical tank has radius 15 feet. Oil begins flowing into the tank in such a way that the depth h of the oil in the tank changes at the rate of 3 ft/hr. At what rate is the top circular surface area of the oil changing when the depth of oil is 8ft. 8. A tank of water in the shape of a cone is being filled with water at a rate of 12 m3/sec. The base radius of the tank is 26 meters and the height of the tank is 8 meters. At what rate is the depth of the water in the tank changing when the radius of the top of the water is 10 meters