Falling charged water drop

A water drop of $18 \times 10^{-3} g$ falls from a charged sphere . The drop's charge is $10^{-9} C$ . There remains $2.5 \times 10^{-6} C$ in the sphere . The radius of the sphere is $20 cm$ . When the drop falls $30 cm$ , what will be its velocity in $ms^{-1}$ ?