A dishonest merchant has 9 identical-looking coins, but one of them is counterfeit and slightly lighter than the rest. Using a standard two-pan balance scale with no weights, what is the minimum number of weighings required to guarantee finding the lighter coin?
2 weighings
Divide the 9 coins into three groups of 3. Weigh group A against group B. If they balance, the fake is in group C. If one is lighter, the fake is in that group. Take the 3 candidate coins and weigh any two against each other; if they balance, the remaining third coin is the fake.
About this riddle
Logic riddles give you a few facts and ask what must be true. List what you know, cross out what cannot be, and see what is left.
Difficulty: Medium. More like this: Logic riddles with answers or try Einstein's Logic Grid for a different kind of puzzle.
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