1.40: Rate of change of volume of a sphere

The volume, $V$, of a sphere of radius $r$ is given by $V = \dfrac{4}{3}\pi r^{3}$. The radius, $r$ cm, of a sphere is increasing at the rate of $0.5$ cm s$^{-1}$. Find, in terms of $\pi$, the rate of change of the volume of the sphere when $r = 0.25$.