So you know that the radius is increasing at the rate of 5 inches per second, so you can write $\frac{dr}{dt}=5$ and similarly we can write $\frac{dh}{dt}=-4$.
We know that the volume of a cone is $\frac{1}{3} \pi r^{2} h$, so to fine the rate of change of volume or $\frac{dV}{dt}$ we need to first express V as $\frac{1}{3} \pi r^{2} h$ and then use the product rule to write $\frac{dV}{dt}$ as something in terms of $r$, $h$, $\frac{dr}{dt}$ and $\frac{dh}{dt}$. Then you use the values of $r$ and $h$ at the values given (30 inches and 20 inches) and you will have found the rate of change of volume at the point where $r=30$ and $h=20$.