Let the weight of the metre scale be ' $W$ ' gf. It will act at the centre of gravity i.e. at $50\,\text{cm}$ mark. According to the principle of moments, in equilibrium, clockwise moment of force $=$ Anticlockwise moment of force $W \times (50-30)\;=40 \times (30-5)$ $\Rightarrow\;\; W \times 20\;=40 \times 25$ $\Rightarrow\;\; W\;=\;\frac{40 \times 25}{20}=50\,\text{gf}$