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Two-Weighing Counterfeit Coin

HardCoding00:00
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Welcome to your interview.

The question is on your right: Two-Weighing Counterfeit Coin. Take a moment with it first.

Talk your thinking through with me if you like - when you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes). Discussion and graded submissions share your five interviewer interactions, so spend them well.

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Problem

You have 8 identical-looking coins, but one is counterfeit and weighs slightly less than the others. How can you find the counterfeit coin using a balance scale only twice?