A 4 kg block A is placed on the top of 8 kg block B  which rests on a smooth table.A just slips on B when a force of 12 N is applied on A. Then, the maximum horizontal force F applied onB to make both A and B move together, is

When force is applied on A, acceleration produced will beFAmA+mB=μmAgmB                                 …(i)When force is applied on B, acceleration produced will beFBmA+mB=μmAgmA                              …(ii)Dividing these two equations, we getFBFA=mBmA∴  FB=mBmA·FA=84×12=24 N