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Marked squares (Posted on 2014-12-24) Difficulty: 3 of 5
Consider a n by n square board, where n is a fixed even positive integer. The board is divided into n^2 unit squares. We say that two different squares on the board are adjacent if they have a common side.
M unit squares on the board are marked in such a way that every square (marked or unmarked) on the board is adjacent to at least one marked square.

Determine the smallest possible value of M.

Source: IMO 1999

See The Solution Submitted by Ady TZIDON    
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  Subject Author Date
SolutionSolutionBrian Smith2016-02-26 12:30:01
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