Mental Math

case math
New answer on Nov 16, 2020
4 Answers
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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
Expert
replied on May 22, 2019
McKinsey Senior EM & BCG Consultant | Interviewer at McK & BCG for 7 years | Coached 350+ candidates secure MBB offers

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

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Lilian on May 22, 2019

Thank you for this!!

Vlad
Expert
replied on May 22, 2019
McKinsey / Accenture Alum / Got all BIG3 offers / Harvard Business School

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!

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Lilian on May 23, 2019

Thanks Vlad.

Clara
Expert
Content Creator
replied on Nov 16, 2020
McKinsey | Awarded professor at Master in Management @ IE | MBA at MIT |+180 students coached | Integrated FIT Guide aut

Hello!

Good question!

Mental math means basically that you won´t be able to use a calculator, no the fact that you cannot write things, etc.

Cheers,

Clara

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Anonymous replied on Aug 22, 2020

Dear Lilian,

I agree with other experts. As a coach, I'm sure that it makes sense to work on your mental math skills during your preparation period.

For practicing and improving your mental math you can find some apps and tools, besides Victor Cheng’s math tool (Magoosh's mental maths app, Mental math cards challenge app, etc).

Consulting math is very different than academic math. Working consultants - and consulting interview candidates - are always under time pressure. Results are what matter and answers are required simply to be good enough to guide business decisions, rather than being absolutely correct.

The next important thing is that rather make it on the paper to structure the notes and then to communicate the results clearly rather than make your calculations fast

The time pressure in case interviews is severe and you cannot afford to waste time. But to make your calculations right you shouldn’t be in a rush. So, I would recommend you here to work on both - practice with time limitations and learn how to keep your mind peaceful and concentrated (it might be not so popular advice here, but mediations really make their job here ).

Be comfortable and confident to state your answers not as a question. Interviewers notice this, and this will not give credit.

If you need any further help or career advice, feel free to reach me out.

Best,

André

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Sidi gave the best answer

Sidi

McKinsey Senior EM & BCG Consultant | Interviewer at McK & BCG for 7 years | Coached 350+ candidates secure MBB offers
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