Question

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

Answer 1

it is by the principle of moments
f1*s1=f2*s2

the distance at which the man with 40gf
sits =20*4/40=80/40=2m

correction to ur question
the weight of the boys must be in kgf not gf

hope it helps