plzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz solve question no.b

Let the maximum force be P. At the moment when P is applied by the man his effective weight on the board reduces to Mg-P. Total force applied on the floor is equal to weight of the man plus weight of the board, i.e.=Mg-P+mg =Normal force by the plane on the board.In this case maximum friction force =µ(Mg-P+mg)This maximum friction force is equal to horizontal pull by the string which in turn is equal to P, equating these two,µ(Mg-P+mg)=P→µP+P=µ(M+m)g→P= µ(M+m)g/(1+µ)