Tricky interview question: You have 10 sets of gold bars and only one is defective. How do you work out which one?

A candidate for an analyst position at a private equity firm in Hong Kong was asked the following question…

The question
You have 10 stacks of gold in front of you. Each stack is made up of 10 gold bars. One of these stacks contains defective bars that all weigh 10.1kg, rather than the standard 10kg

You have scales, but you are only allowed to turn them on once and you can only read the weight on the scales once. How do you find out which gold stack is defective?

The candidate’s reply
I could not provide a definitive answer and gave a probability-based reply which did not impress the interviewers.

How he would have responded in hindsight
I should have taken a minute to think it through. I would place on the scales one bar from stack one, two bars from stack two, and so on. Only when the final 10 bars from stack 10 were loaded would I look at the scales.

The decimal point on the scales would indicate which stack was defective – i.e. if all the gold bars weighted 10kg, then the scales would read 550kg. But if stack seven was defective, they would read 550.7kg.

The question is tricky because it only has one solution, but it also allows scope for the interviewer to see your thought process. It was testing my ability to think through a problem and structure it before diving in, and also my ability to juggle numbers quickly. I didn’t get the job.

How would YOU have answered?
Use the comments box below to suggest a better response to the interview question above.


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