A block A of mass 4 kg is placed on another block B of mass 5 kg, and the block B rests on a smooth horizontal table. If the minimum force that can be applied on A so that both the blocks move together is 12N, the maximum force that can be applied on B for the blocks to move together will be:

In the first case, friction acts only between the 4kg block and the 5kg block.
As a result, if 12N is the force required for both blocks to move together,
In the second case, we consider both the blocks as a system and the force acting between them is an internal force.
Thus the ratio of the forces applied on a block to the mass of the block should be a constant.
12/4​=F​/9
Hence, the maximum force F = 27 N.