5. A water tank (see diagram below) has the shape of a trough whose uniform cross section is the lower half of a circle of radius 10 ft. The length of the trough is 40 ft. The tank is filled with water to a depth of 4 ft. Water is being pumped out from the tank through its top. Find how much work has been done when the depth of the water remaining in the tank is 2 ft. The weight density of water is 62.5 lb. per cubic foot