You have 9 identical-looking gold coins, but 1 is a counterfeit that weighs slightly less than the rest. Using a balance scale, what is the minimum number of weighings required to guarantee finding the fake coin?
2 weighings
Weighing 1: Compare 3 coins against 3 coins. If balanced, fake is in the remaining 3; if unbalanced, fake is in the lighter set. Weighing 2: Take the 3 suspect coins and weigh 1 against 1. If balanced, the 3rd coin is fake; if unbalanced, 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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