A hexagon is cut out of a triangular grid. You have a supply of rombi the size of two adjacent grid triangles with which to tile the hexogon. The rombi come in three varieties each associated with one of the three directions of the common side of their constituent grid triangles. Prove that if the rombi tile the whole hexagon, there is exactly the same number of rombi of each variety.
Instead of tiling the hexagon with rombi, we may think of spliting it into pairs of adjacent triangles. An easy way to do that is to join the centers of any two paired triangles. In the applet below, click on one of the triangles and then move (not drag) the cursor to the other one and click there too. The lines joining the centers may have one of three directions. When no single triangle is left over, there are as many lines in one direction as in any other.
This problem was posted by Guy David and Carlos Tomei in the Proofs Without Words section of the American Mathematical Monthly (v. 96, no. 5, May 1989, pp. 429-430). It was reprinted in [Proofs Without Words, p. 142] and in a concise form in [Winkler, p. 46]. An extended discussion can be found in [The Art of Mathematics, #24].
The problem has been motivated by the form of a French candy and the shape of the box it is usually sold in: