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You are a coin collector who recently acquired a set of rare antique coins. The set contains 14 coins - 12 are authentic but 2 are counterfeit. The real coins each weigh 1 oz, while the fakes each weigh 1.1 oz.

You have an accurate digital scale, but it can only measure the weight of up to 3 coins at a time. What is the minimum number of weighings needed to guarantee you can identify both counterfeit coins?

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    $\begingroup$ Did you make this puzzle yourself? $\endgroup$
    – bobble
    Aug 20, 2023 at 14:15

1 Answer 1

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I thought it was 8. You weight n=[3,3,3,3,2] coins that (in the worst case/ least efficient/ requiring most weighings) they would weigh w=[3,3,3.1,3,2.1] oz.

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