A jeweler has 9 identical-looking gold coins, but one of them is counterfeit and weighs slightly less than the others. Using a two-pan balance scale without weights, what is the minimum number of weighings guaranteed to find the fake coin?
2 weighings
Divide the 9 coins into three groups of 3 (Group A, B, C). Weigh Group A against Group B. If they balance, the fake is in Group C. If one side is lighter, the fake is in that group. Take the 3 coins of the suspect group and weigh two of them against each other. If they balance, the third unweighed coin is fake; if not, the lighter one is 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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