You are handed 9 seemingly identical coins, but one is counterfeit and lighter than the rest. Using a two-pan balance scale, what is the minimum number of weighings needed to guarantee finding the fake?
2
Split the 9 coins into three piles of 3. Weigh pile 1 against pile 2 (weighing 1). If they balance, the fake is in pile 3; if not, it is in the lighter pile. Take the suspicious pile of 3 coins and weigh 1 coin against another (weighing 2). If they balance, the third coin is fake; otherwise, the lighter pan holds the fake.
About this riddle
Math riddles hide a calculation inside a short story. Pull out the numbers, work out what is being asked, and watch for a trick such as a unit change.
Difficulty: Medium. More like this: Math riddles with answers or try Target 24 for a different kind of puzzle.
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