A water tank is box shaped with a square base of side length 10 ft. The tank is fed by a pipe above and drains from its base. In order to have a well mixed solution, both the feeder pipe and the drain must be opened. The tank is being filled at a rate of 10 ft3/s when the height of the liquid in the tank is 15 ft. If the drains are pulling 20 ft3/s, what is the rate of change in the depth of the solution in the tank