A freely falling spherical rain drop gathers moisture (maintaining...

A freely falling spherical rain drop gathers moisture (maintaining its spherical shape all the way) from the atmosphere at a rate $\frac{d m}{d t}=k t^2$ where t is the time and m is the instantaneous mass of the drop, the constant $k=12 g m / \mathrm{s}^3$. If the drop, of initial mass $m_0=2 g m$, starts falling from rest, the instantaneous velocity of the drop exactly after 5 second shall be (ignore air friction and air buoyancy)