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'Weekly Problem 26 - 2013' printed from http://nrich.maths.org/

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This network has nine edges which meet at six nodes. The numbers $1, 2, 3, 4, 5, 6$ are placed at the nodes, with a different number at each node. Is it possible to do this so that the sum of the two numbers at the ends of an edge is different for each edge?
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Either show a way of doing this, or prove that it is impossible.
 
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This problem is taken from the UKMT Mathematical Challenges.

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