Two concentric, conducting, thin spherical shells A and B have radii ‘r’ and ‘R’ as shown in figure. Shell A carries a charge 'q' and B carries a charge ‘Q’. Shell A is supported on an insulating stand. The potential of shell A is

The potential at every point inside the outer shell B is $V_1=\frac{Q}{4\pi {\in }_{0}R}$, which is also the potential on the surface of inner shell A due to outer shell B. The potential on the surface of shell A due to its own charge is $V_2=\frac{q}{4\pi {\in }_{0}r}.$ Therefore, the total potential of shell A is
$V_A=V_1+V_2=\frac{Q}{4\pi {\in }_{0}R}+\frac{q}{4\pi {\in }_{0}r}$
$=\frac{1}{4\pi {\in }_0}\left(\frac{Q}{R}+\frac{q}{r}\right)$