The left mark 3 is 2× 3 = 6m away from the midpoint.
The mark 6 on the right is 6× 2 = 12m from the midpoint.
According to lever theorem
Power× power arm = resistance× resistance arm
If each glass ball is equal in weight
Let x glass balls be placed on the right, and the gravity of each glass ball is1n.
Then 4× 6 = 12x
12x=24
x=2
You need two glass balls on the right to keep balance.
Take it with your hands. thank you