Two concentric spheres of radii R and r have similar charges with equal surface densities ($\mathrm{\sigma }$). What is the electric potential at their common centre

Let Q and q be the charges on the spheres. The potential at the common centre will be
$\begin{array}{r}\mathrm{V}=\frac{1}{4{\mathrm{\pi \epsilon }}_0}\times \frac{\mathrm{Q}}{\mathrm{R}}+\frac{1}{4{\mathrm{\pi \epsilon }}_0}\times \frac{\mathrm{q}}{\mathrm{r}}\\ =\frac{1}{{\mathrm{\epsilon }}_0}\left[\frac{\mathrm{Q}}{4{\mathrm{\pi R}}^2}\times \mathrm{R}+\frac{\mathrm{q}}{4{\mathrm{\pi r}}^2}\times \mathrm{r}\right]\end{array}$
But   $\frac{\mathrm{Q}}{4{\mathrm{\pi R}}^2}=\frac{\mathrm{q}}{4{\mathrm{\pi r}}^2}=\mathrm{\sigma }$
$\therefore \mathrm{V}=\frac{1}{{\mathrm{\epsilon }}_0}[\mathrm{\sigma R}+\mathrm{\sigma r}]=\frac{\mathrm{\sigma }}{{\mathrm{\epsilon }}_0}(\mathrm{R}+\mathrm{r})$