A thin spherical conducting shell of radius fi has a charge q. Another charge Q is place at the centre of the shell. The electrostatic potential at a point P a distance R/2 from the centre of the shell is

The electrostatic potential on the surface of spherical shell of fi having a charge g is given by
${\mathrm{V}}_1=\frac{1}{4{\mathrm{\pi \epsilon }}_0}\cdot \frac{\mathrm{q}}{\mathrm{R}}$
This potential is same at point P. When charge Q is placed at the can be, then potential at P 
${\mathrm{V}}_2=\frac{1}{4{\mathrm{\pi \epsilon }}_0}\cdot \frac{\mathrm{Q}}{(\mathrm{R}/2)}$
$\therefore \text{ Total potential at }\mathrm{P}$
$\begin{array}{l}\mathrm{V}={\mathrm{V}}_1+{\mathrm{V}}_2=\frac{1}{4{\mathrm{\pi \epsilon }}_0}\left[\frac{\mathrm{q}}{\mathrm{R}}+\frac{2\mathrm{Q}}{\mathrm{R}}\right]\\ =\frac{1}{4{\mathrm{\pi \epsilon }}_0}\times \frac{1}{\mathrm{R}}\times (\mathrm{q}+2\mathrm{Q})\end{array}$