Pumping water from a container

A rectangular tank with its top at ground level, is used to catch runoff water. Assume that the water weights 62.4 lb per cubic foot.

Tank dimensions: 10 ft long. 12 ft wide. 20 ft tall.

a. How much work does it take to empty the tank by pumping the water back to ground level once the tank is full?

b. If the water is pumped to ground level with a motor (work output 250 ft-lb/sec), how long will it take to empty the full tank?

I got the right answer to a. by this equation:
work = integral from 0 to 20 times 7488 times (20-y) times dy which equals 1497600

I don't know what to if a motor is thrown into the mix though.