You have 9 identical-looking coins, but 1 is counterfeit and slightly lighter than the others. Using a balance scale with two pans, what is the minimum number of weighings needed to guarantee finding the fake coin?
2
Divide coins into 3 groups of 3. Weigh Group A against Group B. If equal, the fake is in C; if unequal, it's in the lighter group. From that group of 3, weigh 1 coin against another: if equal, the remaining coin is fake; if unequal, the lighter pan holds 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: Hard. More like this: Logic riddles with answers or try Einstein's Logic Grid for a different kind of puzzle.
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