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Croatia Mathematical Competitions

Combinatorics Difficulty 5.3 AIME, harder Prove it Croatia

On the board 8×88 \times 8 tromino-tiles of the shape □\square have to be placed in such a way that each tile covers exactly three cells of the board and the tiles cannot overlap.
What is the least possible number of tromino-tiles that one can place on the board so that no additional tromino-tile can be placed afterwards?

Solution

Let us divide the board into 16 2×22 \times 2 squares as in the picture.
Figure 1
In each of those squares at least two cells have to be covered, otherwise we could place a tromino-tile on three uncovered cells. Hence, at least 32 cells have to be covered, and we need at least 11 tromino-tiles to do that covering.
The following construction shows that 11 tromino-tiles are enough to satisfy conditions of the problem.
Figure 2

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