The distance between 2 charges Qa and Qb is r, and the force between them is F. what is the force between them if the distance is doubled?

The force between them is inversely proportional to the distance between them. (It works exactly like gravity does.)

So if you multiply the distance by 2, then the force gets multiplied by 1/2².

Which is 1/4.

The new force is F/4.

We didn't need to know what the charges or the distance are labeled

in order to answer this question.