You have 9 identical-looking coins, but 1 is a lighter counterfeit. What is the minimum number of weighings required on a balance scale 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 group C; if unequal, it is in the lighter group. From the suspect group of 3, weigh 1 coin against another. If equal, the unweighed coin is fake; if unequal, the lighter coin is 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: Hard. More like this: Math riddles with answers or try Target 24 for a different kind of puzzle.
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