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

If q1  and q2  are the charge on spheres of radius r and R, respectively, in accordance with conservation of charge,Q=q1+q2                         ….. (i)and according to the given problem, σ1=σ2i.e, q14πr2=q24πR2⇒ q1q2=r2R2                                 …… (ii)So, (i) and (ii) giveq1=Qr2(R2+r2)  and q2=QR2(R2+r2)Potential at common centerV=14πε0[q1r+q2R]=14πε0[Qr(R2+r2)+QR(R2+r2)]=14πε0.Q(R+r)(R2+r2)