I really don't understand related rates so I'm having a problem with this question.

The top of a silo has the shape of a hemisphere of diameter 20 feet. If it is coated uniformly with a layer of ice and if the thickness is decreasing at a rate of 1/4 inch/hr, how fast is the volume of the ice changing when the ice is 2 inches thick?

So I basically got to

dr= 1/4 (.25)

dt

When we're looking for dv/dt ? (Volume?)

So I assumed -> 4(pi)r^2(dr/dt) = dv/dt

and I'm pretty lost... :/