Find the ratio of mass of the wooden section to the plastic section of a composite rod pivoted at P

If you consider the composite rod as a whole, the centre of gravity CG is at the pivot. The line of action of the weight which acts from the CG passes through the pivot at P, there is no perpendicular distance, hence no moment created. Thus the rod is balanced at P.

But to solve this question, you need to consider the weight of plastic and weight of wood separately in order to find the ratio and apply the Principle of Moments to solve it.

https://www.youtube.com/watch?v=rG1hHGhqO4w