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Mental math added value



Can someone help me understanding what is the added value in mental math? Btw to be precise it is not math but algebra.

Let me explain myself: when I interview candidates, I do not ask them to calculate mentally the resistivity of a piece of silicon given the data: it is pointless, any computer can do it faster and better. What I want to know from them is if they can explain me what influences the resistivity and if they can provide a physical explanation for that. I give them complex problem, they can use the smartphone if they like, they will not find the answer there. They have to show me how they think and if they are able to come up with innovative solutions.

In other words, my belief is that doing fast calculations in your head is not synonimus of being a math genius: can you tell me how many winners of the Fields medal were very good in fast mental algebra?

But maybe I'm simply wrong.. thanks!

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Anonymous A replied on 08/28/2017


The aim of MBB is not to recruit maths geniuses but people who can compute on the spot (meaning in front of the client) some basic calculations, proving to the said client that:

- consultants are indeed very smart, hence worth its money

- thanks to these basic calculations, consultants can quickly assess the relevance of "innovative solutions" that they may come up with (for instance, in a first meeting with a reluctant client)

- And finally, who would trust the intellect of somebody that cannot do basic mental maths?


Francesco replied on 08/29/2017
Ex BCG | MBB Specialist | #1 Expert for meetings done (900+) and recommendation rate (100%)

Hi Simone,

I agree with the previous comment, mental math is not mainly used as a way to compute how strong on math you are, but rather to analyse three main elements:

  1. What is your math speed under pressure: what is your reaction when in the middle of a revenue brainstorming they ask you “what is 72% of 72”?
  2. How you communicate your thoughts: what is your way to present your analysis/math in few seconds without previous preparation?
  3. What is your emotional reaction when put under pressure, as for your body language: do you close yourself and become hostile, or simply smile and provide the right conclusion with confidence?

This is due to the fact that, unlike other jobs requiring math in the engineering and physics fields, in consulting you have a relevant and continuous client interaction. Mental math questions may thus easily arise with a client in the form of a challenge to your reasoning such as “How can you be sure we will be able to save $5M in just six month?”. In these situations you definitely cannot take your phone or calculator and digit in front of the client or you will lose all your credibility. In short, what they want to test is “What happens if my current client meets this guy alone and challenges him as he is doing with me every day?”

As a consequence, in most cases the kind of math required is also pretty simple. That’s because the interviewer already knows you are smart and can do the math (they screened that when reviewing your CV) and he is more interested in analysing the previous elements.

Hope this helps,

Vlad replied on 08/30/2017
McKinsey / Accenture / More than 300 real MBB cases / Collected all Big 3 offers / Harvard Business School

Completely agree with everything above. Almost every client meeting you have to calculate to calculate something mentally. Moreover, mental maths really helps while checking your team models and deliverables. You can quickly check the numbers and find the mistakes

Simone replied on 08/29/2017

I see your points! It is much more clear now, thanks!