When a boy weighing 20 kgf sits at one end of a 4m long see-saw, it gets depressed at its end. How can it be brought to the horizontal position by a man weighing 40 kgf.

Anticlockwise moment = Clockwise moment

⇒ 20 kgf × 4 m = 40 kgf × d

⇒ d = 20 × 4\over 40 = 1 m

So it is proved that horizontal position can be achieved if a person sits at a distance of 1m from the centre on the side opposite to the boy.