Electric charge question

I tried to do this homework, but my answer is all , of course it is not as same as this question’s answer. That is my answer:
+) First, 2 charged have a same chagre Q, after these also have a same charge q=(Q-Q’).
+) F’/F=[(Q-Q’)/Q]^2.
+) I substituting all answer, and it is correct all, i can have 1 Q’.
Why the answer is C


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