This reasoning concept, a basic deductive rule, concludes a result follows once a statement and its condition both hold.

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Your new ahaModus Ponens

Modus Ponens: Concludes a result follows if a statement and its condition are both true. Source: Modus ponens ↗

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08This number-placement puzzle uses a 9 × 9 grid; each row, column and 3 × 3 box must contain the digits 1–9 once.RECALL
Sudoku
Sudoku requires every digit exactly once per row, column, and box, becoming a global puzzle craze in the 2000s.
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09These word puzzles fit answers to clues into intersecting rows and columns of shared letters.RECALL
Crossword Puzzles
Crossword puzzles link intersecting answers by shared letters, first widely published in 1913.
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10Moving stacked disks between three pegs without putting a larger disk on a smaller one takes at least 2ⁿ − 1 moves in this puzzle.RECALL
The Tower of Hanoi
The Tower of Hanoi takes at least 2ⁿ − 1 moves for n disks. Three disks need 7 moves; four need 15, rather than 14.
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