A 60 kg man pushes a 40 kg man by a force of 60N. The 40 kg man has pushed the first man with a force of                                                              

When two persons A, B push each other there take place action and reaction pair so both exert equal force on each other i.e. in opposite direction.

So force exerted by A on B = $-$ force exerted by B on A

Here it is given 60kg man exert 60N on 40kg man therefore 40kg man will result automatically 60N on 60kg man.