Let man sit at a distance of a m from the centre
If the see saw stands balanced,
See saw is in equilibrium
So, total moment of clockwise force = total moment of anticlockwise force
or, 1.2xgx20 = 60xgxa
or, a = 1.2x20/60 = 0.4
So, man should sit at a distance of 0.4m (40 cm) from the middle of the see saw on the opposite end of the boy.