You have five coins: three are genuine gold coins of equal weight, and two are lighter counterfeit coins of equal weight. Using a balance scale just once, how can you guarantee identifying at least one genuine gold coin?
Place two coins on the left pan and two coins on the right pan, leaving one coin aside.
If the scale balances, both pans contain exactly one fake coin, meaning the unweighed set-aside coin is guaranteed to be real. If the scale tips, the heavier side contains either 0 or 1 fake coin (versus 2 or 1 on the lighter side), meaning both coins on the heavier pan are guaranteed real gold.
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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