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12 Balls, One Odd
CitadelJane StreetTwo Sigma
12 identical-looking balls include exactly one of a different weight — you don’t know if it’s heavier or lighter. Using only a balance scale, what is the minimum number of weighings that guarantees finding the odd ball AND whether it is heavy or light?
Approach
Each weighing gives one of three results.
Count the possibilities you must distinguish: 12 × 2.
Answer
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