The answer is Resonance. Think of the bass on a speaker and if you place a glass of water onto a table or flat surface, it will start to vibrate/ shake because of the musics beats bass.
If two waves with equal amplitude and wavelength travel through a medium such that particular particle is at the crest of one wave and at the trough of another wave, then destructive interference will take place.Both waves will cancel each other out at that point such that a node will form in which the particle will remain at its rest position only and will not move.
Answer:
Explanation:
Given
Wooden board is pivoted at center and
Older child of mass
is sitting at a distance of L from center
if two child of mass
is sitting at a distance
and
(say) from pivot then net torque about pivot is zero
i.e.

as 
Therefore



Therefore another child is sitting at a distance of 