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Trinni is fascinated by triangle numbers (1, 3, 6, 10, 15, 21, etc.) and recently coming across a clock, she found that she could rearrange the twelve numbers 1, 2, 3, ..., 12 around the face so that each adjacent pair added up to a triangle number. She left the 12 in its usual place; what number did she put where the 6 would usually be?
If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.
This problem is taken from the UKMT Mathematical Challenges.View the previous week's solution