You have 9 identical-looking gold coins, but 1 of them is counterfeit and slightly lighter than the rest. You are given a balance scale. What is the minimum number of weighings required to guarantee finding the fake coin?
2 weighings
Divide the 9 coins into 3 groups of 3. Weigh Group 1 against Group 2. If equal, the fake is in Group 3; if unequal, it is in the lighter group. Then take the 3 coins from the lighter group, place 1 coin on each side of the scale. If equal, the unweighed third 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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