If on the concentric hollow spheres of radii  r and R>r  the charge  Q is distributed such that their surface densities are same then the potential at their common centre is

Let say charge q1 is on inner sphere and  charge q2 is on outer sphere  q1+q2=Q  and  q14πr2=q24πR2  (given) q1=Qr2R2+r2 and q2=QR2R2+r2 Potential at common centre=V=V1+V2 =14π∈0Qr2R2+r2r+QR2R2+r2R=QR+r4π∈0R2+r2∵Potential inside shell = potential on the shell