You have 80 identical coins which are all authentic, and also one counterfeit which seems identical to the others but is slightly heavier. You have three balance scales, but one of them is broken and gives random answers. How do you find the counterfeit coin in 7 weighings?

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You have 80 identical coins which are all authentic, and also one counterfeit which seems identical to the others but is slightly heavier. You have three balance scales, but one of them is broken and gives random answers. How do you find the counterfeit coin in 7 weighings?

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