A block of mass 3 kg is at rest on a rough inclined plane as shown in the figure. The magnitude of net force exerted by the surface on the block will be:

Since, the block is at rest under two forces:

(i) weight of block.

(ii) contact force from the plane (resultant of force of friction and normal reaction).

Contact force should be equal to weight (or 30 N) in upward direction. Because under the action of two forces, a body remains in equilibrium when both the forces are equal in magnitude, but opposite in direction, i.e. 30 N.