(i) Weight mg (W) of rule produces an anti-clockwise moment about the knife edge O. In order to balance it, 20gf must be suspended at the end B to produce clockwise moment about the knife edge O.
(ii) From the principle of moments,
Anticlockwise moment = Clockwise moment
W × (58-50) = 20gf × (100 − 58)
W × 8 = 20gf × 42
W = 20gf x 42/8 = 105 gf