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'Four in a Row' printed from http://nrich.maths.org/
In the game Four-in-a-Row, two players take it in turns to place counters on the $5 \times 5$ board. The winner is the first player to have $4$ adjacent counters in a line across or down (but not diagonally).
It is Black's turn to play next. Where should she play her fourth counter to be certain of winning on her fifth turn whatever White plays?