You have 9 identical-looking coins, one of which is slightly heavier than the rest. Using a balance scale, what is the minimum number of weighings needed to guarantee finding the heavy coin?
2 weighings
Divide the 9 coins into three groups of 3. Weigh two groups against each other. If they balance, the heavy coin is in the unweighed group; if not, it is in the heavier group. Take that group of 3 and weigh 1 coin against 1 coin to find the heavy coin in the second weighing.
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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