I wrote: <<

PUZZLE: Given a rhombus tiling of a 2n-gon (so that each polygon edge is a rhombus edge), show that the number of rhombi is determined by n.

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As Michael Kleber and George Hart have (perhaps inadvertently) pointed out, I neglected to mention (except in the Subject line) that the 2n-gon is supposed to be regular. For a straightforward generalization to parallelograms, one need only assume that the 2n-gon is convex and has n pairs of parallel sides. --Dan