The two charges are then brought closer together so that the force between them increases by a factor of 25. By what factor was their separation decreased

Explanation:

The forces between two charges is inversely proportional to the square of distance.

So  if the distance is half  then the force increases by a factor of 4 . if force between the charges increases by a factor of 25 then separation between them is decreased by a factor of  5 i.e distance between them is 1/5.    

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