You have 9 coins that look identical, but one is counterfeit and weighs slightly less than the rest. Using a balance scale, what is the minimum number of weighings needed to guarantee finding the fake coin?
2 weighings
Divide the 9 coins into 3 groups of 3. Weigh Group A against Group B. If equal, the fake is in C; if unequal, it is in the lighter group. From that lighter group of 3, weigh 1 coin against 1. If equal, the unweighed 3rd coin is fake; if unequal, the lighter coin 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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