Two concentric spheres of radii R and r have similar charges with equal surface densities (σ). What is the electric potential at their common centre

Let Q and q be the charges on the spheres. The potential at the common centre will beV=14πε0×QR+14πε0×qr=1ε0Q4πR2×R+q4πr2×rBut   Q4πR2=q4πr2=σ∴ V=1ε0[σR+σr]=σε0(R+r)