Let place the charge Q on the line between +4q and q charges as shown on the figure.
Then, according to the Coulomb's low, the interaction force between +4q and Q charges is:
The interaction force between +q and Q charges is:
where is the distance between +4q and +q and is the Coulomb's constant.
In the equilibrium state both forces are equal, thus obtain the equation:
.
Solving it for , obtain:
Answer. The charge Q should be placed at the distance 2/3r from the charge +4q.
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