Torque balance for rotational equilibrium Steps:

• Torque from 40 N weight: 40 N × 20 cm = 800 N·cm clockwise.
• Balancing torque must equal 800 N·cm counterclockwise.
• For position 40 cm right, required force F satisfies F × 40 cm = 800 N·cm, so F = 20 N.
• Downward direction at right produces counterclockwise torque to oppose the clockwise torque.

Why B is correct:

• 20 N downward at 40 cm right creates 20 N × 40 cm = 800 N·cm counterclockwise torque, equaling the original torque per the principle of moments (∑τ = 0).

Why the others are wrong:

• A: Upward at right creates clockwise torque, worsening the imbalance.
• C: Downward at left adds 50 N × 10 cm = 500 N·cm clockwise torque.
• D: Upward at left creates only 50 N × 10 cm = 500 N·cm counterclockwise torque, insufficient for balance.

Final answer: B