Here's a challenge: to place on the chessboard of the minimum number of rooks so that each box was attacked by no less than 2 rooks. The rook can attack through the other boat and does not attack the field on which stands.
Answer: Twelve at least.
1 2 3 4 5 6 7 8 ___ ___ ___ ___ ___ ___ ___ ___ h | | | | | A | A | A | | --- --- --- --- --- --- --- --- g | A | | | | | | | | --- --- --- --- --- --- --- --- f | A | | | | | | | | --- --- --- --- --- --- --- --- e | A | | | | | | | A | --- --- --- --- --- --- --- --- d | | | | | | | | A | --- --- --- --- --- --- --- --- c | | | | | | | | A | --- --- --- --- --- --- --- --- b | | | | | | | | A | --- --- --- --- --- --- --- --- a | | A | A | A | | | | | --- --- --- --- --- --- --- ---
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