Twenty seven drops of same size are charged at $220\text{V}$ each. They combine to form a bigger drop. Calculate the potential of the bigger drop.

Let $r$ and $q$ be the radius and charge of each small drop.Potential $V=\frac{kq}{r}=220\text{V}$.When $27$ drops combine, the volume of the big drop is $27\times$ volume of small drop.
$$\frac{4}{3}\pi R^3=27\times \frac{4}{3}\pi r^3\Rightarrow R=3r$$
Total charge on the big drop $Q=27q$.Potential of big drop $V^{\prime }=\frac{kQ}{R}=\frac{k(27q)}{3r}=9\frac{kq}{r}$ $
$V^{\prime }=9\times 220=1980\text{V}$ $