A growing sand pileSand falls from a conveyor belt at the rate of 10 m^3 / min onto the top of a conical pile. The height of the pile is always three-eighths of the base of the diameter. How fast are the (a) height and (b) radius changing when the pile is 4m high? Answer in cm/min

I've set up the picture as best as I can. I'm trying to understand the next step.

I first need to find the the rate of change of the height, or dh/dt... so would I differentiate both sides, giving me dV/dt = 1/3π[ 2r(dr/dt) + r^2(dh/dt)] ? I am not sure if this is right or how to proceed.