Is quick mental math a skill that can be learned?

fast math
Edited on Sep 05, 2021
8 Answers
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Anonymous A asked on Oct 26, 2019

I want to be true to myself. I think I have what it takes to be a consultant, but my problem is math. I am not quick and when somebody stairs at me while asking a math question is like if all my insecurity comes out and just blocks me. I panic and can't do the easiest multiplication (let alone the divisions).

I have two months ahead: do you genuinely think I can make it? Can I learn how to be quick at math?

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Francesco
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updated an answer on Oct 28, 2019
#1 Expert for Coaching Sessions (3.600+) | 1.300+ Reviews with 100% Recommendation Rate | Ex BCG | 8+ Years of Coaching

Hi Anonymous,

it seems your problem is not the math itself, but the math under pressure. This is a very common problem for many candidates.

The majority of the people are actually fine with math when there is no pressure.

However they “suddenly” make a lot of mistakes in a case: missing zeros, simple math errors in divisions, wrong percentages, etc.

Why is that the case?

I have coached people in thousands of case interviews and I found there are two main reasons why candidates make mistakes under pressure:

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1) Practicing math without a time constraint.

You may have a PhD in Math, but unless you practice math with a tough time constraint during the prep for consulting interviews, you will likely do mistakes.

Why? Because in a consulting interview you will face something you haven’t practiced before: pressure.

You know you are judged for each mistake you do and that you have to find a solution quickly.

This makes easy to do mistakes.

Unless you have trained for that in advance.

What is then a good solution for that?

Practice math with a time constraint that is almost unfeasible for you to satisfy initially.

Eg: if the average time you spend for percentage exercises is – say – 15 seconds, give yourself slightly less – say 12 seconds – for the following exercise.

Not only you will learn how to speed up your math, but forcing yourself to solve something in an “almost impossible” time will create pressure and will help you to get accustomed to it.

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2) Not presenting a structure before doing the math.

Many people start their math exercise jumping to the numbers.

This is not the best approach to follow.

You should treat math in the same way you deal with the structure at the beginning of the case.

You don’t start a case going in details in one area, but presenting the overall approach you have, and THEN moving to the details.

In the math part you should do the same thing: present first your theoretical approach (the formula you are going to use), and THEN move to the numbers.

You may wonder how this may help to avoid math mistakes - in the end, you still have to go through the math anyway.

The reason is simple: your math mistakes in a case are usually not related to lack of math knowledge, but to the pressure you feel.

Presenting first your theoretical approach and aligning with the interviewer will not only help to understand if your approach is correct, but also to release pressure.

Indeed, once presented your approach, you know before the actual math if your logic is correct or not.

This usually makes also your math easier due to the fact you know you are following the right path.

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If you exercise on the previous two points I am pretty sure you can improve quite a lot your math skills before your interview.

Hope this helps,

Francesco

(edited)

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Ian
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replied on May 04, 2020
BCG | 100% personal interview success rate (8/8) and 95% candidate success rate | Personalized interview prep

100%. 2 months is plenty of time.

I found the following useful

  • Online "Drills": https://www.jetpunk.com/quizzes/fast-math-multiplication-quiz
  • Math sheets (print these and do them on paper): https://www.math-drills.com/

Also, remember to practice applying fast math in a case. This includes reading charts and receiving information (i.e. writing it down differently based on whether it's clearly 1 dimensional or multi-dimensional information)

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Thomas
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replied on May 03, 2020
150+ interviews | 5+ years experience | Kearney & Accenture | Sold consulting startup| London Business School

Absolutely, there are many tricks to learn to improve your mental math. I would recommend having a look at the basics of numbers to better understand how numbers interact. Additionally, practice 5 mins of mental math a day really helps.

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Antonello
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replied on Oct 26, 2019
McKinsey | MBA professor for consulting interviews

Hi,

2 months can be enough if you work well.

Here a video for fast multiplications (the youtube channel has lots of other math tricks): https://www.youtube.com/watch?v=PYrgjMubh-c

Best,

Antonello

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Anonymous replied on May 06, 2020

Dear A,

It is possible. I can provide you more resources to practice your mental math

Good luck!

Best,

André

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Vlad
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replied on Oct 27, 2019
McKinsey / Accenture Alum / Got all BIG3 offers / Harvard Business School

The following skills will speed up your calculations and reduce the probability of making a mistake:

1) Learn how to solve equations and systems of equations. As you could have noticed, some cases require solving an equation. You can use GMAT books to practice.

2) Learn how to multiply double-digit numbers really fast. It takes just a couple of hours to learn how to apply this method on paper and a couple of days to start doing these calculations mentally, but it's worth it. Please follow the link for more details: https://www.youtube.com/watch?v=6ndkkPZYJHo

3) Learn how to work with zeros.

Simply always use the powers of 10. Then you'll be able to multiply / divide the numbers and sum up / subtract the powers separately:

For example: 300 x 9000 = 3 x 10ˆ2 x 9 x 10ˆ3 = 3 x 9 x 10ˆ(2+3) = 27 x 10ˆ5 or 2.7 Mln

If you get used to writing all the numbers that way, you will never lose zeros.

4) Learn the division table. This method will help you calculate any percentage problems like market shares or margins. For example, if your market is $620M and your revenues are $5.1M you can use 5/6 or 83.3%, as a proxy to calculate the market share. By adjusting to zeroes and slightly decreasing the number, you'll get 8.2%

Best

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Marco-Alexander
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updated an answer on Sep 05, 2021
Former BCG | Case author for efellows book | Experience in 6 consultancies (Stern Stewart, Capgemini, KPMG, VW Con., Hor

(edited)

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

Hello!

Yes, it can. I wuold suggest you to practice with GMAT.

GMAT unfortunately only gets better with practicing. Good news is that there are many ways of doing so!

There are free exams in the internet that you can use for practice (the one of LBS MBA page, Verits prep, as well as some free trials for courses such as the one of The Economist (https://gmat.economist.com/)

Hope it helps!

Cheers,

Clara

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Francesco gave the best answer

Francesco

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