You have 7 identical-looking coins, but 1 is a heavier counterfeit. What is the minimum number of balance scale weighings required to guarantee identifying the counterfeit coin?
2 weighings
Weighing 1: Compare 3 coins vs 3 coins (leave 1 aside). If balanced, the unweighed coin is fake (1 weighing). If unbalanced, take the 3 heavier coins. Weighing 2: Compare 1 coin vs 1 coin from those 3. If balanced, the remaining coin is fake; if unbalanced, the heavier side is fake.
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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