The radius of a right circular cone is increasing at a rate of 5 inches per second and its height is decreasing at a rate of 3 inches per second. At what rate is the volume of the cone changing when the radius is 50 inches and the height is 40 inches?

I went to get help with this problem and I was advised to differentiate the Volume equation in terms of r and h and plug in the numbers. But I didn't get the right answer. I did this:

$\displaystyle V=\pi r^2(t)h(t)$

$\displaystyle dV/dt = 2\pi r(t)r'(t)h(t)+h'(t)\pi r^2(t)$

Plugged in the numbers and got $\displaystyle 12500\pi$, which isn't right!

What am I doing wrong?