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It is a silly question - but, What's the role of mental math & brain teasers in interviews?

Anonymous A asked on Oct 22, 2018 - 5 answers


I know that these are silly questions. But, I hope you can spare a minute to entertain this silliness.

  • Why is it important to have a good mental math "power" when we have our calculators in our phones?
  • How do brain teasers help in interviews?
  • AND; are there any tips to approach mental math or brain teasers in a more efficient way?

I would love to hear from the experienced people here.

Generally speaking, I am a bit on the slow side. It takes me few seconds to solve a 2 digit by 2 digit problem. It would take me even longer to do the "more complex" ones. That is a problem for me.

I am practicing more and more and hoping to gain back the skills that I have long lost.

I appreciate your input and feedback.

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replied on Oct 22, 2018
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On a related note:

I have always had very positive feedback from interviewers when I told them that I already knew the answer to a given brainteaser, instead of pretending to solve it right then and there.

Bonus points for honesty and backbone, I guess. :-)

replied on Oct 22, 2018
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Hi Anonymous,

good question, I have asked myself this quite often.

I think we have to view the two things separately:

Mental Math

Yes, we do have calculators on our phone. So for a precise answer, we can always refer to the powerful computing machines around us. Mental math mostly helps to get a feeling for "Wait a second, that doesn't add up" - so to identify at which point it's time to turn on the number cruncher. It's also helpful to be able to do a bit of mental math in moments when you can't easily get out your phone and do the math - while you're presenting something, or in a no-phones meeting.

But I would really be interested in seeing scientific evidence that good mental math performance is a good predictor of job performance.

Brain teasers

Brain teasers, in general, are more a test for creativity and outside of the box thinking. Of course, there are different flavours, some more math-y, some more creative. I guess they test with how many approaches to a solution you can come up with, whether you can look beyond the obvious, whether you can think on your feet, whether you can "feel" your way to a solution. This may help you during your work because if the solution to a problem were obvious, you wouldn't call the highly paid consultants, would you now?

But again, I haven't seen any scientific evidence that brain teaser chops translate into good consulting skills.



Hi Elias. As always. Thank you for the very useful insights and commentary. — Mike (Mustafa) on Oct 23, 2018 (edited)

replied on Oct 22, 2018
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There is no dedicated part of the interview for brainteasers and to be honest they are quite rare. Although the interviewer might want to ask you a brainteaser question either before or after the case. To prepare you simply read the 2-3 most popular books with the brainteasers (Are you smart enough to work at Google?, How would you move Mount Fuji?).


HI Vlad. Thanks for the tips. An I like the one with moving Mount Fuji. Cheers. — Mike (Mustafa) on Oct 23, 2018

Tobias replied on Nov 07, 2018
Engineer with Leadership experience in Non Profit

in general I believe it is not mainly about the ability to do mental math.
It is about the competences you show while doing these estimation tasks.

For example the ability to:

  • Think fast
  • Structure yourself in an understandable way
  • Make sound assumptions to solve your problems

Of course it is also about precision - imagine how MC Kinsey would look like in front of their customers if one of their consultants is making several math mistakes in a conversation. It all falls back to the company.

I hope it makes sense.

All the best,

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updated his answer on Oct 22, 2018
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Hi Anonymous,

I agree with Vlad, brainteasers are not very common (referring to my experience, when I applied, I got 1 out of 20+ interviews). Still, answering to your question, why do they ask them?

The main reason is to understand how you structure complex problems. Brainteasers usually require a more out-to-the-box approach that is more difficult to prepare than market sizing or cases. As a consequence they let the interviewer understand some peculiar elements of the logic of the candidate even when he/she is extensively prepared on cases.

The negative side of brainteasers is that they are usually a one-dimension type of question - that is, you need one key inside only to solve it. Business cases and market sizing include instead more key elements to be analyzed and thus offer more insides on the candidate.

Mental math is a different topic. You have to know how to do mental math in your head, as you could have to do it in front of the client. You cannot have a math problem and tell the client "Please give me one second, I will calculate 67% of 67 on my phone". Part of the skills you have to show to clients is credibility, and good mental math is one of the elements that will support that.

To improve on brainteasers, the best thing would be to download a list of the most common ones and simply practice on them - you will recognize after a while there are some commonalities between them. A good idea would be to do it gradually, allocating them proportionally in the days before the interview to keep learning constant. As mentioned, they are pretty unlikely nowadays though.

To improve on the math, you can consider the following tips:

  • Divide complex math in smaller logical steps: if you have to compute 96*39, you can divide it in 96*40 - 96*1 = 100*40 - 4*40 - 96*1 = 4000 – 160 – 100 + 4 = 3744
  • Use shortcuts: the most useful to know are:
    • Fractions, at least 1/6, 1/7, 1/8, 1/9
    • Cubes, ideally until 19^2
  • Practice: there are several online services you can find to improve your speed; I would recommend to keep track of the performance in an excel sheet to assess whether you are improving and if not, analyze the reasons for that




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