$mg=F\Rightarrow mg=\frac{k{{\left| q \right|}^{2}}}{{{r}^{2}}}\to m\times 10=\frac{9\times {{10}^{9}}\times 16\times {{10}^{-12}}}{{{(0/4)}^{2}}}\Rightarrow m=9\times {{10}^{-2}}kg$