Let the weight of the metre scale be ' W ' gf. It will act at the centre of gravity i.e. at 50‌cm mark.
According to the principle of moments, in equilibrium,
clockwise moment of force = Anticlockwise moment of force
W×(50−30)‌=40×(30−5)
⇒‌‌W×20‌=40×25
⇒‌‌W‌=‌

40×25

20

=50‌gf