We are given a cylindrical tank with radius of 5m. The tank is being filled at a rate of 3 cubic meters per minute, and we are asked to find the rate of change of the height of the water.

The volume at time t is `V(t)=pir^2h(t)` so `(dV)/(dt)=(pir^2)h'(t)` since r is a **constant**.

The solver could just have easily have used `V=25pih` so `(dV)/(dt)=25pih'(t)` . Why didn't they? The method used can now be generalized to right cylindrical tanks of any given radius -- just substitute the given radius for r.