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Berkeley Math Circle Monthly Contest 8

Combinatorics Difficulty 5.2 AIME, harder Prove it United States

Problem:
Determine whether it is possible to tile a standard 8×88 \times 8 chessboard with 1515 LL-tiles and 11 TT-tile of the shapes below:

Figure 1

(Each tile covers four squares of the chessboard. The tiles can be flipped and rotated at will.)

Solution

Solution:
Let the squares of the chessboard be colored black and white in the usual way. Note that the LL-tile always covers two squares of each color, while the TT-tile covers either 11 black and 33 white or 33 black and 11 white squares. Since the chessboard has equal numbers of black and white squares, it is impossible to cover it with these tiles.

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