Two concentric spherical shells of radii R and 2R carry charges Q and 2Q respectively. Change in electric potential on the outer shell when both are connected by a conducing wire is $\left(\mathrm{K}=\frac{1}{4{\mathrm{\pi \epsilon }}_0}\right)$

Before connection potential on outer surface of outer shell
${\mathrm{V}}_1=\frac{1}{4{\mathrm{\pi \epsilon }}_0}\left[\frac{\mathrm{Q}}{2\mathrm{R}}+\frac{2\mathrm{Q}}{2\mathrm{R}}\right]=\frac{1}{4{\mathrm{\pi \epsilon }}_0}\left[\frac{3\mathrm{Q}}{2\mathrm{R}}\right]$
After connection charge Q goes to outer sphere.
Potential on outer surface of outer shell, total charge on the outer sphere is 3Q
${\mathrm{V}}_2=\frac{1}{4{\mathrm{\pi \epsilon }}_0}\left[\frac{3\mathrm{Q}}{2\mathrm{R}}\right];{\mathrm{V}}_1-{\mathrm{V}}_2=0$