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This question already has an answer here:

You have 9 coins. One of the nine is counterfeit. The counterfeit coin can be distinguished by weight - it is heavier than the rest.

Using a balance scale only twice, find the counterfeit coin.

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marked as duplicate by Oray, JMP, Glorfindel, Bass, APrough Jan 15 '18 at 16:06

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ I think I've seen this puzzle here somewhere. $\endgroup$ – ibrahim mahrir Jan 15 '18 at 12:27
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    $\begingroup$ Which is already a duplicate's duplicate btw. $\endgroup$ – Alix Eisenhardt Jan 15 '18 at 13:21
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Ternary search.

Split into groups of 3. Call them first3group, second3group, third3group.

Now weigh first3group vs second3group. If first test shows equal weights, the heavy coin is in third3group. Split third3group and weigh 2. Where one is heavier, QED, otherwise, it is the non-weighed one, QED.

Else take the heavier group of 3 and do a similar exercise as described above when left with 3, 1 of which is heavier.

QED

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