Two concentric spherical shells have masses `M_1, M_2` and radii `R_1, R_2 (R_1 lt R_2)`. The force exerted by this system on a particle of mass m. If it is placed at a distance `((R_1+R_2)/2)` from the centre.

Explanation

The gravitational force on .m. due to the shell of `M_2` is zero.
The gravitational force on .m. due
`=(GM_1m)/((R_1+R_2)/2)^2 =(4GM_1m)/(R_1+R_2)^2`