This should help you get a feel for the pool problem, number 26 in the related rates section. Here is my solution to the problem in pdf. Play around with the slider, if you hit the little arrow in the lower left it will animate. You should be thinking of the volume and the height both as functions of time.
To find the volume, you need to use the width of the pool (20 ft) and the area of a trapezoid whose top edge is the top edge of the water.