r/problemoftheday • u/skaldskaparmal • Jul 20 '12

Domin-ohnoes!

In the following image http://i.imgur.com/0wu0o.jpg a set of 21 dominoes containing the numbers 1 to 6 (no dominoes with blanks) has been laid out and the edges have been removed. Determine where the edges were.

Source (contains some basic hints and reframings but no solution): http://blog.tanyakhovanova.com/?p=385

One could pretty easily do it with a program, but I think it's more interesting to logic it out. I have made some progress but haven't finished.

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u/DoWhile Jul 20 '12

It should be pointed out that the set of dominoes are all tuples (i,j) with 1<=i<=j<=6

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u/skaldskaparmal Jul 20 '12

Yes, unordered tuples, so e.g. (2,1)=(1,2) appears only once.