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B. 12 N

Explanation:

Static Equilibrium

Static equilibrium occurs when an object is at rest, i.e., neither rotating nor translating.

In the static rotational equilibrium, the total torque is zero with respect to any rotational axis.

The torque applied by a force F perpendicular to a displacement X with respect to a reference rotating point is:

T = F*X

Considering the pivot as the rotating point, there are 3 forces producing torque. Let's consider counterclockwise torques as positive, thus:

30N * 40 cm - 60 N * 30 cm + F (30 cm + 20 cm) = 0

Operating:

1200 N.cm - 1800 N.cm + F(50 cm) = 0

- 600 N.cm + F(50 cm) = 0

Solving for F:

F = 600/50

F = 12 N

B. 12 N