Hi,I'm having some trouble with a related rate problem. A pool is 40ft long, 20 ft wide, 4 ft deep at the shallow end and 9 ft deep at the deep end. Water is being pumped into the pool at 10 ft^3/min. How fast is the water level rising when there is 4 ft of water in the deep end?

I've tried using volume of a trapezoidal prism and differentiating that, but it didn't seem to work. Any help is appreciated. Thanks a lot.