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Cvent Software Developer Interview Question (student candidate)

I interviewed in McLean, VA and was asked:
"Brainteaser: 10 bags of coins. 1 bag has fake coins. Each normal coin weighs 10g. Each fake coin weighs 11g. Using one scale and one weighing, how do you find the fake bag of coins?"
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Part of a Software Developer Interview Review - one of 143 Cvent Interview Reviews

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3
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Take 1 coin from the 1st bag, 2 coins from the 2nd bag, 3 coins from the 3rd bag, etc... and weigh them together. If there are 10 bags of normal coins, you know that the total weight would be S = (1+2+3+ ... + 10)I*10. If the fake bag is the 3rd bag, then the total weight would be 30 + S and you would know it's the 3rd bag. By looking at the difference between the total weight and S, you know which bag it is.
- Interview Candidate on Apr 28, 2012

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