Hello fellow mathletes!

I'm not sure if this is in the right forum but I have come across a related rates question in my textbook and just cannot figure it out.

A spherical storage tank has a radius of 15m. Water is being pumped in at a rate of 36pi m^3/min. Find the rate of depth increase at the instant that the water level is 6m below the very top of the sphere.

HINT: rate of depth increase = (rate of inflow)/(surface area of liquid)

Thanks in advance and look forward to solving your problems,

Katy