Here we have another optimization problem. Remember the key is to come up with the formula for what we are optimizing and take the derivative.
We assume the balloon is spherical which gives us V = 4/3πr3
Now we take the derivative (using the chain rule):
dV/dt = dV/dr * dr/dt = 4πr2 * dr/dt
Now we just plug in numbers!
dV/dt = 4π(10)2 * 2 = 800π in3/sec