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Two point charges X and Y, exert a force F on each other when they are at a distance d apart (x and y are opposite charges). When the distance between them is 20mm, the force exerted on each other is 0.5F. What is the distance d?

I know that, e.g. doubling r in F proportional to 1/r^2 will make F be 1/4 of the original, and tripling will make F 1/9 of the original but I do not know how it works the other way at all.

It looks like the relationship would be r proportional to square root of 1/F... I do not understand this relationship.

Could someone explain this in a very simple and understand way? I would also prefer this done as mathematically as possible...

I am so troubled please I have an exam tomorrow and I just cannot understand how to get F from d value for 0.5F...