[TFBloom]

Sorry you found the text confusing; often I have reworded the problem text to give what is, for me, the clearest explanation of the problem.

Of course, what the clearest explanation is is very much dependent on one's own background and personal tastes!

For example for this problem I think a lot about sumsets and difference sets anyway, so it made sense to phrase it in those terms.

If you have a suggestion on how it could be rephrased for clarity then please do email in.