A metre rule balances when the 50 cm mark is directly above a pivot. State where in the rule its centre of mass is located?

The ruler's center of mass is located on the vertical line defined by the 50 cm mark.

Explanation:

Recall that in order to have equilibrium as that described by this example, the net Torque in the ruler should be zero. which means in our case that the weight due to the right half of the ruler, should equal in absolute value of the weight of the left side of the ruler, thus defining where the ruler's center of mass is.

Since this happens at the mark 50 cm, then the center of mass of the ruler is located somewhere inside the ruler somewhere above where the pivot is located (50 cm mark)

15.3 cos 65 degrees trig