what is mass of metre time which is in equilibrium

Let the mass of the ruler be m. 

From the given diagram and given information it is clear that the midpoint is 50 cm 

Thus, 

Clockwise moment = (50 -30) x m = 20 x m

Anticlockwise moment = (30 - 5) x 40 = 25 x 40 

 

By principle of moments, 

Clockwise moment = Anticlockwise moment

Thus, 

20 x m = 25 x40

m = 50 g 

 

Hence, the mass of the ruler is 50 g