Two concentric, thin metallic spheres of radii R1 and R2(R1>R2) bear charges Q1 and Q2 respectively. Then the potential at distance r between R1 and R2 will be (k=14πϵ0)
Potential due to sphere of radii R2 is V2=kQ2r (as r outside of the sphere)
Potential due to sphere of radii R1 is V1=kQ1R1 (as r inside of the sphere and potential inside metal sphere is constant).
Total potential Vr=V1+V2=kQ1R1+kQ2r=k(Q1R1+Q2r)