You have 12 identical-looking coins. Eleven weigh the same, but one is counterfeit and has a different weight (either lighter or heavier). Using a balance scale, what is the minimum number of weighings required to guarantee finding the fake coin and determining if it is light or heavy?
3 weighings
Using decision tree logic, group the coins into sets of 4-4-4. The first weighing compares two sets of 4. Depending on whether it balances or tilts, you narrow down the set containing the fake and its possible relative weight, allowing complete resolution in 2 additional weighings.
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: Hard. More like this: Logic riddles with answers or try Einstein's Logic Grid for a different kind of puzzle.
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