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Electric fields question-proving Coulombs law

I'm tutoring an A-level physics student, and probably a bad sign but I can't answer this question. They have asked to go over a set of questions given to them in one of their lessons. This was the last one in their worksheet, maybe because i've spent a good 45 minutes working through these questions I can't think straight anymore. Can anyone give any clues as to how they can calculate the force between these two "balloons", given the information here?