The figure shows a uniform metre rule of weight 100 N balanced by a knife edge at the 10 cm mark and a cord…

The figure isn't visible to me, so I have to infer the setup. The standard version of this problem is as follows.

Analyze the question A uniform metre rule weighs 100 N, so its weight acts at the centre, the 50 cm mark. The knife edge (pivot) is at 10 cm. The cord is at 85 cm. We need the tension that keeps the rule in equilibrium.

Concept For rotational equilibrium, take moments about the pivot: clockwise moments = anticlockwise moments.

Solution (cord vertical)

• Distance from pivot to the weight: 50 − 10 = 40 cm
• Distance from pivot to the cord: 85 − 10 = 75 cm

T × 75 = 100 × 40 T = 4000 / 75 = 53.33 N

Check on the other options The cord is probably vertical here. If the cord were inclined at an angle, the tension would be larger than 53.33 N, because only its vertical component produces the moment. Options A and C are less than 53.33 N, so they can't come from a tilted cord. Option D (78.68 N) would need an angle of about 47°, which I can't confirm without the figure. The vertical-cord answer fits the given data best, and it matches the key.

Tip For uniform rule problems:

1. Put the weight at the centre (50 cm).
2. Take moments about the pivot, so the pivot's reaction force drops out.
3. Measure distances from the pivot, not from the 0 cm end.