Principle of moments Steps:

• Fulcrum at 15 cm; 50 g mass at 23 cm (8 cm right); 20 g mass at 11 cm (4 cm left).
• Moment from 50 g: 50 × 8 = 400 g·cm (clockwise).
• Moment from 20 g: 20 × 4 = 80 g·cm (counterclockwise).
• Net clockwise moment: 400 - 80 = 320 g·cm; balance requires m × d = 320, where d = 8 cm to 0 cm mark (per diagram). Why B is correct:
• 40 g × 8 cm = 320 g·cm counterclockwise, equaling net moment per principle of moments (∑ clockwise moments = ∑ counterclockwise moments). Why the others are wrong:
• A. 30 g: 30 × 8 = 240 g·cm, insufficient to counter 320 g·cm.
• C. 47 g: 47 × 8 = 376 g·cm, overcompensates the net moment.
• D. 133 g: 133 × 8 = 1,064 g·cm, far exceeds required balance.

Final answer: B