KöMaL 2011 B4392
There are chessmen standing on a few fields of a 20×20 chessboard. A chessman can be removed from the board if at least one half of the fields in its row or column are vacant. What is the minimum number of chessmen needed for an appropriate arrangement such that no chessman can be removed from the board?
Official text KöMaL. Quoted with source.
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