Volume of water in the tank:
Differentiate both sides with respect to time <em>t</em> :
<em>V</em> changes at a rate of 2000 cc/min (cubic cm per minute); use this to solve for d<em>h</em>/d<em>t</em> :
(The question asks how the height changes at the exact moment the height is 50 cm, but this info is a red herring because the rate of change is constant.)