Let the two children be sitting on the left arm. They will produce anti-clockwise moment due to their weights about the middle point.
Total Anti-clockwise moment = 30kgf × 2m + 50kgf × 2.5m
= 60kgfm + 125kgfm = 185kgfm
To balance it, the man should sit on the right arm (to produce clockwise moment). Let his distance from the middle be x m. Then