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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? –  Zeta.Investigator Aug 27 '12 at 16:31
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I know the answer, but im mainly interested with the source :) –  Timothy Lu Aug 27 '12 at 16:32
    
1 4 7 9 10 is for 5 –  Euler....IS_ALIVE Aug 27 '12 at 16:34
    
If you add 5 to the set $\,\{1,4,7,9,10\}\,$ you have $\,9-5=5-1=4\,$... –  DonAntonio Aug 27 '12 at 16:37
    
The first question just said "give an example of a LUCKY set with five elements." Which I did... –  Euler....IS_ALIVE Aug 27 '12 at 17:31

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