Water is being pumped into a vertical cylinder of radius 4 meters and height 15 meters at a rate of 2 meters^{3}/min. How fast is the water level rising when the cylinder is half full?
$\displaystyle V = \pi r^2 h$
since radius is constant ...
$\displaystyle V = 16\pi h$
you are given $\displaystyle \frac{dV}{dt}$ , take the time derivative of the volume equation and determine $\displaystyle \frac{dh}{dt}$ ... you'll find that the amount in the cylinder doesn't matter.