Question

A meter rule is pivoted at its mid point c with a vertical force of 10N hanging from the distance 30cm from c at what distance must a15N force hang to balance the ruler horizontally

Answer 1

$$\tau_{clockwise}=\tau_{anti-clockwise},$$ $$F_1r_1=F_2r_2,$$ $$r_2=\dfrac{F_1r_1}{F_2}=\dfrac{10\ N\cdot30\ cm}{15\ N}=20\ cm.$$