V=4πr³/3 is the volume V of a sphere of radius r.
It’s not clear from the question what the units are, but let’s assume the volume is in cm³, then the radius will be in cm.
So dV/dt=4πr²dr/dt where dV/dt=50 cm³/s is the rate of change of the volume and dr/dt that of the radius in cm/s.
Therefore, 4πr²dr/dt=50.
If the initial radius is 10cm, then 400πdr/dt=50 and dr/dt=1/(8π) cm/s. If π=3.142 the rate of change of the radius is about 0.04cm/s.