I don't understand how you are supposed to do this

Question

Helium is being pumped into a spherical balloon at a rate of 8 cubic centimeters per second. How fast is the radius of the balloon increasing when the radius is 7 centimeters? Round your answer to four decimal places.

Answer 1

Volume V = 4πr³/3. Then dV/dt = 4πr²dr/dt; dr/dt = (dV/dt)/(4πr²) = (8cm³/s)/(4π·49cm²) = 0.0130 cm/sec