You have 9 identical-looking coins, but one is a counterfeit that is lighter than the other 8 genuine coins. Using a balance scale only twice, how can you guarantee finding the fake coin?
Divide the coins into 3 groups of 3. First, weigh group 1 against group 2. If balanced, the fake is in group 3; if unbalanced, the fake is in the lighter group. Second, take the 3 coins from the lighter group and weigh 1 against 1. If balanced, the unweighed coin is fake; if unbalanced, the lighter coin is fake.
By splitting the 9 coins into 3 groups of 3, a single balance comparison narrows the search to 3 candidate coins. A second comparison among those 3 coins pinpoints the lighter counterfeit coin with certainty.
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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