Given that h=r/2, the volume formula is
V = pi r^3 / 6
dV/dt = dV/dr * dr/dt
dV/dr = pi r^2 / 2
When h = 40in, r = 80in so dV/dr = 3200 pi in^2
r = 2h so dr/dt = 2 dh/dt = 6 in/min
dV/dt = (3200 pi in^2)(6 in/min) = 19200 in^3/min
So your answer is correct but your units are slightly off.