IMO Shortlist 2012 problem C5


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3. studenoga 2013.
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The columns and the row of a 3n \times 3n square board are numbered 1,2,\ldots ,3n. Every square (x,y) with 1 \leq x,y \leq 3n is colored asparagus, byzantium or citrine according as the modulo 3 remainder of x+y is 0,1 or 2 respectively. One token colored asparagus, byzantium or citrine is placed on each square, so that there are 3n^2 tokens of each color.
Suppose that on can permute the tokens so that each token is moved to a distance of at most d from its original position, each asparagus token replaces a byzantium token, each byzantium token replaces a citirine token, and each citrine token replaces an aspargus token. Prove that it is possible to permute the tokens so that each token is moved to a distance of at most d+2 from its original position, and each square contains a token with the same color as the square.
Izvor: Međunarodna matematička olimpijada, shortlist 2012