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Mental Math

Lilian asked on May 22, 2019
Aspiring Management Consultant

I have a question about the meaning of mental math...

What exactly is considered mental math in a case interview?

Can looking at numbers, separating them into easier to calculate numbers and noting down the bits and pieces on paper so you don't forget and then adding them all up in your head pass for mental math?

I am asking cause I wonder if I am expected to keep all the strings I separate in my head and not write them down at all and recall them smoothly and only write the final digits down.

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Sidi
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replied on May 22, 2019
McKinsey Engagement Manager & BCG Consultant | Interviewer at McK & BCG for 7 years | Coached 90+ candidates secure MBB offers
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Hi Lilian!

This obsession with "Mental Math" is one of the many damaging nonsense myths that are flying around! Mental math is simply not something that is required in ANY way!

  • You are not tested for doing math stunts in your head, but for the rigor and correctness of your analysis.
  • You will NOT get bonus points for being lightning fast!
  • You WILL get bonus points for being very clear and easy to follow.

This means, the skill you have to build is to quickly identify what you need to analyze and why, and organizing this in a way that you can work through this in a very linear and easy to follow manner. Hence, I recommend to ALWAYS use pen and paper for your analysis (e.g., using tables works very well in many situations).

At the extreme, being exceptionally fast in mental math will even HURT your case performance! The reason is simple: the interviewer will always assume that the way you behave in an interview is also how you behave vis-a-vis a client. If you become a human calculator, doing all sorts of complicated math stunts lightning-fast in your head, then you become VERY HARD TO FOLLOW. Hence, the quality of your communication deteriorates. And this is always your fault, not the client's!

So in essence, make sure that you are doing clear and very easy-to-follow calculations, using pen and paper to write down the required steps first and then to perform the actual computation. Trying to becme a lightning fast human calculator has a lot of risks and practically no benefit.

Cheers, Sidi

Thank you for this!! — Lilian on May 22, 2019

Vlad replied on May 22, 2019
McKinsey / Accenture / Got all BIG3 offers / More than 300 real MBB cases / Harvard Business School
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Hi,

Mental math is about 3 things:

  1. Having no mistakes
  2. Calculating in an efficient way
  3. Doing it fast

If you are doing all 3 using calculations on paper - that's fine.

Several skills may be useful here:

  • Learn how to multiply double digit numbers (https://www.youtube.com/watch?v=6ndkkPZYJHo)
  • Learn the division table up to 1/11 (i.e. 5/6 = 83.3)
  • Learn how to work with zeros (Hint: 4000000 = 4*10ˆ6)
  • Use math tools (Mimir math for iOS), Math tool on Viktor Cheng website to practice

Best!

Thanks Vlad. — Lilian on May 23, 2019

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