You have four identical keys labeled A, B, C, and D. Three weigh 10 grams each, but one is a lighter counterfeit weighing 9 grams. Using a balance scale once, you weigh (A + B) against (C + 10g standard weight). The scale balances perfectly. Which key is the counterfeit?
Key D
Since the scale balances, both sides weigh exactly 20g. That means A, B, and C must all be genuine 10g keys. By elimination, Key D is the 9g counterfeit.
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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