Wednesday, May 8, 2019

My Thoughts (on results as asker): Interview question: 9 balls, find one odd with a balance scale

Disclaimer: This post does not contain the answer the question, although this may contain small hints.

I have posed this question to several people, and found it interesting the common logical point people get stuck on. The most common answer I get is three. Out of 20 or so people (rough estimate), only 1 person (math teacher) figured this out on their own.

Majority of the responses is that they put four on each side. If equal, then the one on the table. If not, take the group that is heavier. Put two on each side, then one on each side.

After they have exhausted their thoughts, I would clue them by asking why they choose four. Why not one, or two, or three?

What I found most interesting is that majority of the people still cannot come up with the answer of two. The reason I find this very interesting is because they use similar logic when they put four balls on each side. The other interesting oddity is even if they brute force the answer (by trying one on each side, then two on each side, then three on each side), some still cannot come up with the answer.

As an Interviewer

Although I enjoy asking this more like a party game, I do not think this is a very effective question for an interview. The question does not effectively get the interviewee to talk out loud, even after I tell them that they should think out loud.

Perhaps they do not want to appear unintelligent by going through wrong assumptions even though everyone does it as part of the process. Or perhaps they don't want to appear to be using brute force.

Maybe a more reasonable assumption is that it is much faster to go through it in your head than talking out loud since the samples are simple enough. Unlike more out-of-the-box questions like how many golf balls fit in a Boeing 747 or monkey with a sombrero, where the question naturally gets interviewees to talk out loud.

Since the interview already is time limited, my personal opinion is that there are better questions to ask if I am trying to test someone's ability to think through a problem. Although


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