Math CentralQuandaries & Queries


You are considering buying 27 silver coins that look alike, but you have been told that one of the coins is a lightweight counterfeit. Find the least number of weighings on a balance scale that you can use to be certain you have found the counterfeit coin.

Hi Gary,

Here is a technique I would try. Divide the coins into three piles, at least two of which have the same number of coins. Put the two equal size piles on the scale. If they balance then the light coin is in the pile that you didn't weigh. If they don't balance then the light coin is on the side of the scale that was the lightest.

I hope this helps,

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