'$n$' small spherical drops of the same size which are charged to 'V' volt each coalesce to form a single big drop. The potential of a big drop is

Let the radius of each small drop be $r$ and its charge be $q$.

The potential of each small drop is $V = \dfrac{1}{4\pi\epsilon_0}\dfrac{q}{r}$.

When $n$ such drops coalesce, the total charge of the big drop is $Q = nq$.

The volume is conserved, so the volume of the big drop is equal to the sum of the volumes of the $n$ small drops.

$\dfrac{4}{3}\pi R^3 = n \left(\dfrac{4}{3}\pi r^3\right)$

$R^3 = n r^3$

$R = n^{\frac{1}{3}} r$

The potential of the single big drop is:

$V' = \dfrac{1}{4\pi\epsilon_0}\dfrac{Q}{R}$

$V' = \dfrac{1}{4\pi\epsilon_0}\dfrac{nq}{n^{\frac{1}{3}} r}$

$V' = n^{1 - \frac{1}{3}} \left(\dfrac{1}{4\pi\epsilon_0}\dfrac{q}{r}\right)$

$V' = n^{\frac{2}{3}} V$