I'm New at math C and this question is bugging me, can any1 solve this equation so i know how to do it?

The question says:

The rate of loss of water from a rainwater tank with a leak in the side depends on the pressure at the hole. The pressure depends on the height of water above the leak.

A cylindrical tank has a leak 20cm from the top, which has not been fixed because it is only a short distance form the top. Five hours after the tank is filled, the level has dropped by 15cm. Using a differential equation, develop a model to describe the height of water above the hole as a function of time. If another hole is made 20cm from the bottom of the tank, which is 2 metres high, how long will it take before half the water has leaked away?