Two free point charges +q and +4q are a distance L apart. A third charge is placed so that the entire system is in equilibrium. Find the sign, magnitude, and location of the third charge. Below is the picture that my instructor gave me.
What I don't understand is that how is he able to find that r31 = L/3 ?
Second question is that why did he picked so that the net charge on the first force is zero? why not choose another one? Is it just a matter of choice?