Hi dear,
They should kept a distance of √3 times of d. Here is solution
F=k.q1.q2/d^2
Let they kept at a distance x then force is F1=k.q1.q2/x^2.
Now put F1=F/3 I.e. F1=k.q1.q2/3.d^2
Now solve it you will get x=√3d.
Hope it helps.
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