If n drops of mercury, each charged to a potential V, coalesce to form a single drop, the potential of the big drop will be 

If Q is the charge on each small drop, charge on the big drop is ${\mathrm{Q}}^{\mathrm{\prime }}=\mathrm{nQ}. \text{ Now }{\mathrm{Q}}^{\mathrm{\prime }}={\mathrm{C}}^{\mathrm{\prime }}{\mathrm{V}}^{\mathrm{\prime }}=\mathrm{Q}=\mathrm{CV}$. Therefore

                     $\frac{{\mathrm{V}}^{\mathrm{\prime }}}{\mathrm{V}}=\frac{{\mathrm{Q}}^{\mathrm{\prime }}}{\mathrm{Q}}\times \frac{\mathrm{C}}{{\mathrm{C}}^{\mathrm{\prime }}}=\frac{\mathrm{n}}{{\mathrm{n}}^{1/3}}={\mathrm{n}}^{2/3}$