A balloon which always remains spherical,

Let r be the radius and V be the volume of the spherical balloon at any time t. Then, 

$\mathrm{V}=\frac{4}{3} \pi r^{3}$

$\Rightarrow \frac{d V}{d t}=4 \pi r^{2} \frac{d r}{d t}$

$\Rightarrow \frac{d r}{d t}=\left(\frac{1}{4 \pi r^{2}}\right) \frac{d V}{d t}$

$\Rightarrow \frac{d r}{d t}=\frac{900}{4 \pi(15)^{2}}$                 $\left[\because r=15 \mathrm{~cm}\right.$ and $\left.\frac{d V}{d t}=900 \mathrm{~cm}^{3} / \mathrm{sec}\right]$

$\Rightarrow \frac{d r}{d t}=\frac{900}{900 \pi}$

$\Rightarrow \frac{d r}{d t}=\frac{1}{\pi} \mathrm{cm} / \mathrm{sec}$