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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.