Water is flowing at a rate of 50 m^3/min from a shallow concrete conical reservoir, vertex down, of base radius 40m and height 6m. How fast in cm per minutes, is the water level falling when the water is 3ft deep?

I know the volume of a cone is V = (pi/3)r²h

and I know to use the chain rule: dV/dt = (dh/dt)(dV/dh)

but I don't know how to find dh/dt or dv/dh