The constant of proportionality is k, V is the volume, S is the surface area. Remember that you must use the prime notation for derivatives, not the Leibniz notation, in your answers.
A raindrop evaporates at a rate that is proportional to the surface area: $= 0$.

Since $V = \frac{4}{3}\pi r^3$ and $S = 4\pi r^2$ we have by the chain rule that $\frac{dV}{dt} =$ . Substituting into the original differential equation and solving we find that
$\frac{dr}{dt} =$ so that $r =$