A boy is sitting at a distance of 1.5 m from one end.
Plank of see-saw is 5 m long.
The fulcrum of the see saw will be exactly at centre of plank i.e. 2.5 m
Thus, load arm is 2.5 - 1.5 = 1m
Load arm x load = effort arm x effort
1 x 40 = EA x 25
EA = 1.6 m
Thus, the distance at which a girl must sit from other end is = 2.5 - Effort arm = 2.5 - 1.6 = 0.9 m