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How many queens (queens) do you need to attack the whole chessboard?

Answer: Five. For example:

  1 2 3 4 5 6 7 8
  ___ ___ ___ ___ ___ ___ ___ ___
 H |  | | |  | | |  | | |  | | |  | | |  | | |  | | |  | | |
  --- --- --- --- --- --- --- ---
 G |  | | |  | | |  | | |  Q |  | | |  | | |  | | |  | | |
  --- --- --- --- --- --- --- ---
 F |  | | |  | | |  | | |  Q |  | | |  | | |  | | |  | | |
  --- --- --- --- --- --- --- ---
 E |  | | |  | | |  | | |  Q |  | | |  | | |  | | |  | | |
  --- --- --- --- --- --- --- ---
 D |  | | |  | | |  | | |  Q |  | | |  | | |  | | |  | | |
  --- --- --- --- --- --- --- ---
 C |  | | |  | | |  | | |  | | |  | | |  | | |  | | |  | | |
  --- --- --- --- --- --- --- ---
 B |  | | |  | | |  | | |  | | |  | | |  | | |  | | |  | | |
  --- --- --- --- --- --- --- ---
 A |  | | |  | | |  | | |  Q |  | | |  | | |  | | |  | | |
  --- --- --- --- --- --- --- ---