this question is giving me a really hard time...

A spherical drop of radius r(t), r cms, t minutes evaporates at a rate proportional to its surface area. Given that the volume v(t)=4/3╥r^(3) and the surface area s(t)=4╥r^(2), write down an ODE for r(t).

I am very stuck i have no idea of where to begin, can anyone please help me, Thank you