How fast is the balloon's radius growing when the radius is 5 cm?

Explanation

1. 01

   $\frac{dV}{dt} = 4\pi r^{2}\frac{dr}{dt}$, so $100 = 4\pi(25)\frac{dr}{dt}$.

2. 02

   $\frac{dr}{dt} = \frac{100}{100\pi} = \frac{1}{\pi}$ cm/s.