Interview Question

Interview(Student Candidate) McLean, VA

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?

Interview Answer

1 Answer


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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