Here's another one that I also find difficult in terms of setting it up:
Sand is poured onto the top of a conical pile at a rate of 100m^3/minute. The coefficient of friction of the sand is such that the height and radius are always the same. At what rate is the height increasing when the pile is 12 meters high? The formula for the volume of a cone is V = 1/3pir^2h, but since the height and radius are the same this can be rewritten as V = 1/pih^3.
Again, I'm pretty much lost. I vaguely understand that the dV/dt is related to the dh/dt, but I'm not sure how to make it work.