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In 80 coins one coin is counterfeit. What is minimum number of  weighings to find out counterfeit coin?

PS: The counterfeit coin can be heavy or lighter.

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    It can't be done with weighings at all. No self-respecting counterfeiter would commit the rookie mistake of getting the mass wrong.2012-10-20
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    Please specify if you know whether the coin is heavy or light. That changes the process a bit.2012-10-20
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    @Ross The question doesn't state that the counterfeit coin has a different weight from the others. My guess is that the real coins are made of gold and the counterfeit one, although it weights the same, is made of brass, and can be distinguished from the real coins with no weighings at all.2012-10-20
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    @MJD: Good point. I can't imagine how to do it with less than zero weighings.2012-10-20

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