MathCastle
Proof Writing & Problem SolvingSuperchallenge

Proof Writing & Problem Solving · Superchallenge problem

A ring puzzle has 1010 rings in a row, numbered 11 to 1010, each either ON or OFF. Ring 11 can always be switched (ON to OFF or back). For k≥2k \ge 2, ring kk can be switched only when ring k−1k - 1 is ON and rings 1,…,k−21, \ldots, k - 2 are all OFF. One move switches one ring. Starting with all rings ON, what is the smallest number of moves that turns all of them OFF?

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