Twelve balls, one heavier or lighter, and three weighings on a balance to find it and say which.

Twelve identical balls, one fake that is heavier or lighter, and a balance you can use three times. The story shows the scale tipping left, right and staying level, then asks how to find the fake and its direction. You pick one of four answers. Then you step through the answer and the breakdown.

Why quant interviews test this

Saying the 24-versus-27 count out loud before any weighing is what interviewers listen for. The written solution is at /brain-teasers/twelve-balls/solution.

The Twelve Balls and a Scale guide covers how scoring works, the strategy that wins, a worked example and the mistakes most players make.

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