HMMT 2004 U44

Shown on your answer sheet is a $20 \times 20$ grid. Place as many queens as you can so that each of them attacks at most one other queen. (A queen is a chess piece that can move any number of squares horizontally, vertically, or diagonally.) It's not very hard to get 20 queens, so you get no points for that, but you get 5 points for each further queen beyond 20 . You can mark the grid by placing a dot in each square that contains a queen.