A uniform half metre rule balances horizontally on a knife edge at 29 cm mark when a weight of 20 gf is suspended from one end.

(i) Diagram of the arrangement is shown below:

(ii) If W gf is the weight of half meter rule, by the principle of moments,

$$\begin{aligned}W \times (29 - 25) = 20 \times (50 - 29) \\[0.5em] \Rightarrow W \times 4 = 20 \times 21 \\[0.5em] \Rightarrow W = \frac{20 \times 21}{4} \\[0.5em] \Rightarrow W = 105 \text{ gf}\end{aligned}$$