Triangular clock

Trinni rearanges numbers on a clock face so each adjacent pair add up to a triangle number... What number did she put where 6 would usually be?
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Problem



Trinni is fascinated by triangular numbers (1, 3, 6, 10, 15, 21, etc.).

She found that she could rearrange the twelve numbers on a clock face so that each adjacent pair added up to a triangular 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.