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I was hoping someone could tell me if they knew the source to this problem:

Let S be a subset of {1, 2, 3, 4,..., 10, 11}. We say that S is LUCKY if no two elements of S differ by 4 or 7.

The questions are: Give an example of a LUCKY set with five elements, and is it possible to find a LUCKY set with six elements? Explain why or why not.

This was a really beautiful problem and so I was wondering if someone could tell me the source. I saw this problem like 3 years ago I believe on yahoo answers.

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    I noticed that too, which makes me even more curious to find this source.2012-08-27

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