A charge Q is distributed over two concentric hollow spheres of radii r and R (R >r) such that the surface densities are equal. Find the potential at the common centre.

$\frac{1}{4\pi\epsilon_0}\frac{Q(R+r)}{R^2+ r^2}$

$ q_1 + q_2 = Q$  .......(1)

$ \sigma = \frac{q_1}{4\pi r^2} = \frac{q_2}{4\pi R^2}$ ..............(2)

From (1) and (2) 

$q_1 = \frac{Qr^2}{R^2 + r^2}$

$ q_2 =  \frac{QR^2}{R^2 + r^2}$

$V_c = V_1 + V_2 = \frac{1}{4\pi\epsilon_0} (\frac{q_1}{r} + \frac{q_2}{R}) = \frac{1}{4\pi\epsilon_0}\frac{Q(R+r)}{R^2+ r^2}$