What is gravitational potential energy of system consisting of two concentric spherical shells of mass, m, and 4 m and radii R and 2R.

Total energy equals sum of self energy of two shells and their mutual energy.

self energy$-\frac{{\mathrm{Gm}}^2}{2\mathrm{R}}-\frac{\mathrm{G}16{\mathrm{m}}^2}{4\mathrm{R}} \mathrm{and} \mathrm{mutual} \mathrm{potential} \mathrm{energy} -\frac{\mathrm{G}4\mathrm{mm}}{2\mathrm{R}}$