You have 9 visually identical gold coins, but one is counterfeit and slightly heavier than the other 8. You have a two-pan balance scale. What is the minimum number of weighings needed to guarantee identifying the fake coin?
2 weighings.
Divide the 9 coins into three groups of 3. Weigh Group 1 against Group 2. If equal, the fake is in Group 3; if unequal, it's in the heavier group. From the suspect group of 3, weigh 1 coin against another. If equal, the unweighed 3rd coin is fake; if unequal, the heavier 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: Medium. More like this: Logic riddles with answers or try Einstein's Logic Grid for a different kind of puzzle.
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