rate of change

Let h cm be the height of the heap of sand
Let V cm3 be the volume of sand
V=1/3 (π)(h tan 30°)2h
dV/dt = 1/3(π tan230°)3h2(dh/dt)
dh/dt = 3 dV/dt ÷[(π tan230°)3h2]
dh/dt∣h=15 = (102 x 3)÷[π(tan230°) x 3 x 15 x 15]
dh/dt∣h=15 = 2/(15π) cm min-1