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This lesson takes approximately 15 minutes to complete. It includes 22 interactive questions with immediate feedback.

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LessonMathematicsIntermediate

Grandpa Never Loses — How to always win the 21 matchstick game (Nim)

The matchstick game — 21 matches on the table, take one, two or three, and whoever picks up the last match wins — is not luck at all: it is completely solved, and this lesson shows you how to work the winning rule out yourself. You start where the game ENDS, at zero matches, and count backwards until you find the trap numbers: 4, 8, 12, 16, 20. Then you find out the thing that took a kid eleven summers to notice — why Grandpa always went first.

15 min22 questionsGrades 4, 5, 6

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