Question:

A solid sphere of mass M and radius a is surrounded by a uniform concentric spherical shell of thickness 2a and mass 2M. The gravitational field at distance 3a from the centre will be:

2GM9a2

GM3a2

2GM3a2

GM9a2

Correct option is B. GM3a2
We use Gauss's law for gravitation.
∮g.4πr² = 4πG(Mass enclosed)
g = 3M/4πG
4π(3a)² = MG/3a²