Improving Math Skills

case math
New answer on Dec 14, 2020
7 Answers
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Anonymous A asked on Dec 06, 2020

I'm currently preping for MBB/T2 case interviews and would like to know how to improve the following math skills

  1. Problem structuring, such as how to set up the equation to solve when given a bunch of information (currently going through Insead case book and doing the math but wondering if there are more effective ways or better case books to do this to)
  2. Mental Math
    1. dividing by two digit numbers e.g. 4094/63
    2. multiplication with large numbers 156 x 982 (is this even necessary or should 2x2 be sufficient?
    3. dealing with large number of 0s, sometimes I am able to divide the number correctly in the above examples but I don't know how many digits to actually put

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Francesco
Expert
Content Creator
replied on Dec 06, 2020
#1 Coach for Sessions (4.000+) | 1.400+ 5-Star Reviews | Proven Success (➡ InterviewOffers.com) | Ex BCG | 9Y+ Coaching

Hi there,

For what concerns mental math: you can always write down the math unless the interviewer explicitly asks to do the math in your head, which is quite rare.

In terms of how to approach math in the case, this is what I would recommend:

  1. Repeat the question – sometimes candidates do mistakes answering the wrong question
  2. Ask for time and present how you would like to proceed from a theoretical point of view
  3. Perform the math and present the interim steps to keep the interviewer aligned – don’t just say the final number
  4. Continue with the computations until you find the final answer
  5. Propose next steps on the basis of the results you found

In terms of general math tips and avoiding mistakes, I would recommend the following:

  1. Use correctly 10^ powers in your math computation. For example 3.2B/723M can be transformed in 3200*10^6/732*10^6, which makes it easier to deal with math
  2. Ask if it is fine to approximate. When you have to deal with math in market sizing - and sometimes even in business cases - you are allowed to approximate math to simplify the computation. In the previous example you could transform the computation in 320*10^7/70*10^7
  3. Keep good notes. One of the reasons people do mistakes with big numbers is that they don't keep their notes in order and forget/misreport numbers
  4. Divide complex math in logical steps. This is something you can use for big numbers after the application of the 10^ power mentioned above. For example: (96*39)*10^6 à 96*40 - 96*1 = 100*40 - 4*40 - 96*1 = 4000 – 160 – 100 + 4 = 3744*10^6
  5. Use shortcuts for fractions. You can learn by heart fractions and thus speed up/simplify the computation - the most useful to know are 1/6, 1/7, 1/8, 1/9.

I would also recommend to practice math under pressure - not just math. Many candidates are totally fine doing 67% of 67 in normal conditions, but freeze if you ask this suddenly in a case interview.

In order to do so, try always to use a timer with a strict time constraint when you practice math – this will create pressure and help to replicate the actual conditions of the interview.

Hope this helps,

Francesco

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Vlad
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replied on Dec 06, 2020
McKinsey / Accenture Alum / Got all BIG3 offers / Harvard Business School

Hi,

As for your 1st question - the best way is to download all available case books and go just through the math problem. There is a limited number of types of problems in cases and you can cover almost all of them

Basically, you need to develop 3 calculation skills:

1) Learn how to multiply double digit numbers (https://www.youtube.com/watch?v=6ndkkPZYJHo).

2) Learn how to work with zeros. Best way - always use 10^power instead of zeros

Example:

300x9000 = 3*10ˆ2 x 9*10ˆ3=3x9*10ˆ(2+3)=27*10ˆ5

Handwritten it looks not that complicated. If you get used to writing all the numbers that way, you will never lose zeros and all multiplications/divisions will be replaced with + or -.

3) Learn the division table up to 1/11 (i.e. 5/6 = 83.3%). It will help you calculate any percentage problems

Use math tools to train (Preplounge tool, Mimir math for iOS, Math tool on Viktor Cheng website). Train, train, and train again

Good luck!

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Henning
Expert
replied on Dec 06, 2020
Bain | passed >15 MBB interviews as a candidate

Hi A!

many candidates face the same challenge as you. Rocketblocks has a great set of drills that are tailored to the type of problems you're dealing with in interviews. For a free resource, check out the math sections of GMAT prep materials (e.g. GMATclub).

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Antonello
Expert
Content Creator
replied on Dec 14, 2020
McKinsey | MBA professor for consulting interviews

Hi,
In interviews the aspect which causes more errors is pressure: start to solve calculations with strict time constraint. For longer formulas always share the calculation structure with the interviewer before starting to write down the numbers: this helps to take time, to reduce the pressure, and gives you the opportunity to receive a first feedback from the interviewer avoiding wrong calculations.

I recommend practicing with:
- Preplounge math tool: https://www.preplounge.com/en/mental-math.php
- Youtube channels with math tricks:
https://www.youtube.com/playlist?list=PLjbxBzUM6SLlx_x8UABAdAB7Rbg9feqYa

Here you can find some useful links to simulate PST:
- https://mconsultingprep.com/free-practice-mckinsey-pst/
- https://igotanoffer.com/pages/mckinsey-problem-solving-test
- https://www.mckinsey.com/~/media/McKinsey/Careers%20REDESIGN/Interviewing/Main/
Problem%20Solving%20Test%20PDFs/practice-test-A.ashx

Here some free link to practice with the quantitative part of SHL test
- https://www.gradtests.com.au/test/starttest/TestType/2/TestId/45/practice/1
- https://www.wikijob.co.uk/aptitude-test/verbal-reasoning-practice
- https://www.practiceaptitudetests.com/sign-up/programme/shl
- https://www.shldirect.com/en/practice-tests

Best,

Anto

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Clara
Expert
Content Creator
replied on Dec 08, 2020
McKinsey | Awarded professor at Master in Management @ IE | MBA at MIT |+180 students coached | Integrated FIT Guide aut

Hello!

I like the way you have broken it down there!

For point 1, unfortunately, only gets better with practicing (good cases) & with good feedback. Have you considered a coach for this?

For point 2, also practice on the 3 sub points.

Overall for math strenght, that would tackle both your points 1&2, I would strongly recomment you practice it with the Integrated Reasoning part of the GMAT exam.

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/)

Furthermore, you can leverage the MBB tests (https://www.myconsultingoffer.org/case-study-interview-prep/bcg-online/, https://www.psychometricinstitute.co.uk/Free-Aptitude-Tests.asp, and many others)

Hope it helps!

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Gaurav
Expert
Content Creator
replied on Dec 07, 2020
Ex-Mckinsey|Certified Career Coach |Placed 500+ candidates at MBB & other consultancies
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Ian
Expert
Content Creator
replied on Dec 07, 2020
MBB | 100% personal interview success rate (8/8) and 95% candidate success rate | Personalized interview prep
  1. 100% Recommend Rocket Blocks. It's exactly what you're looking for https://www.rocketblocks.me/
  2. Online "Drills": https://www.jetpunk.com/quizzes/fast-math-multiplication-quiz
  3. Math sheets (print these and do them on paper): https://www.math-drills.com/
  4. In addition to that, you can ask other PrepLoungers to case you on math-heavy cases. You can also search for those case types here and work through them yourself.

Some key formulas/concepts:

  • Breakeven
  • NPV (with + without growth, perpituity + 1-2 years from now)
  • % Change
  • ROI
  • Margin
  • Markup
  • Inventory turnover

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Some great answers from a variety of angles have already been asked. Check these out!

Mental Math

https://www.preplounge.com/en/consulting-forum/mental-math-help-7962

https://www.preplounge.com/en/consulting-forum/is-quick-mental-math-a-skill-that-can-be-learned-5210

Conceptual/Contextual Math

https://www.preplounge.com/en/consulting-forum/materials-for-practicing-conceptual-case-math-8016

https://www.preplounge.com/en/consulting-forum/math-concepts-6951

Key Math Equations

https://www.preplounge.com/en/consulting-forum/math-equations-their-use-7934

Math Practice

https://www.preplounge.com/en/consulting-forum/case-math-practice-6877

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Francesco gave the best answer

Francesco

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