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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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    So...You know the answer and you just want to find the SOURCE?2012-08-27
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    I know the answer, but im mainly interested with the source :)2012-08-27
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    1 4 7 9 10 is for 52012-08-27
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    If you add 5 to the set $\,\{1,4,7,9,10\}\,$ you have $\,9-5=5-1=4\,$...2012-08-27
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    The first question just said "give an example of a LUCKY set with five elements." Which I did...2012-08-27
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    What exactly do you mean by *the* source? [Googling the text](http://www.google.com/search?q=%22We+say+that+S+is+LUCKY+if+no+two+elements+of+S%22) yields [a *Yahoo! Answers* thread](http://answers.yahoo.com/question/index?qid=20090211203039AAZEwpI) and a couple of PDFs, but I suppose you already know that -- are you looking for a book or something where these people got the problem from?2012-08-27
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    Yes, I am looking for a book that would have this problem2012-08-27
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    Interestingly, the PDFs that Google brings up are all related to the 2011 [Michigan Mathematics Prize Competition](http://mmpc.svsu.edu/index.html), but that was after the problem turned up on Yahoo! Answers, so they must have taken it from somewhere. I wouldn't have expected a math competition to use known problems from public sources.2012-08-27
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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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