List of top Physics Questions on System of Particles & Rotational Motion asked in Manipal

In an equilateral triangle $A B C, F_{1}, F_{2}$ and $F_{3}$ are three forces acting along the sides $A B, B C$ and $A C$ as shown in the given figure. What should be the magnitude of $F_{3}$ so that the total torque about $O$ is zero?