If the depth of water is d, then the radius of th surface is r=d/sqrt(3) since

the semi vertical angle is 30 degrees. So the volume of water is:

V = (1/3) pi r^2 d = (1/3) pi d^3/ sqrt(3)

(volume of cone formula used)

Now we are told that dV/dt = 100 pi cc/min. so we differentiate the formula

for V wrt t:

dV/dt = [1/(3 sqrt(3)] pi 3 d^2 dd/dt

so when d=5 cm we have:

dd/dt = 4 sqrt(3) cm/min

which is the rate of change of depth.

RonL