Five sacks of coins, one sack all fake, and one reading on a digital scale to find it.

Five sacks of 100 coins. Real coins weigh 10 g, and every coin in the one fake sack weighs 9 g. A digital scale gives one reading. The story asks how to find the fake sack. You pick one of four answers. Then you step through the answer and the breakdown.

Why quant interviews test this

The idea is to make each possible answer look different. Expect the binary follow-up. The written solution is at /brain-teasers/counterfeit-coins/solution.

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

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