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Reasoning through quant problems (and getting better quickly)?

Sabrina asked on Sep 10, 2019 - 4 answers
Preparing for interviews with MBB, Bridgespan, OW -- and others.

Hi all,

Sabrina here -- Management student ramping up for cases. In isolating the areas that I have the biggest struggles is 1) quant overall, but 2) specifically formulaic parts of quant problems.

Does anyone have advice on how to improve quant, and reasoning through formulaic problems (like break-even, NPV, etc), without prior knowledge on those formulas?


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replied on Sep 10, 2019
Former BCG | Case author for efellows book | Experience in 6 consultancies (Stern Stewart, Capgemini, KPMG, VW Con., Horvath, zeb)
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Hello Sabrina,

I would recommend you three things:

1. Write all standard formulas together (break-even, NPV, etc) and memorize them. With that a large part is already won!
2. For developing unknown formulas in the case situation, you can practice with math exercise books (it is a mixture of solving equations and text exercises in math).
3. If you have issues with mental arithmetic, you can practice very well here:

Best regards

replied on Sep 10, 2019
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The best approach is to download all casebooks available (tons of them) online and go specifically through math in the cases.

Here is a set of things you need to know:

1) Math and formulas:

  • Interest rates calculations
  • Compound margin
  • NPV using the Rule of 72, Perpetuity
  • CAGR using the Rule of 72
  • Currencies exchange calculations
  • Speed, time, distance calculations
  • Equations, systems of equations
  • Ratio & Proportions
  • Little's Law
  • Solving the problems by calculating the area of the triangle
  • Profit / breakeven formula
  • Correlations, outliers (being able to spot on the graphs, tables)

2) Fast math skills

  • Rounding up and down
  • Learn how to multiply double digit numbers (
  • Learn the division table up to 1/11 (i.e. 5/6 = 83.3)
  • Learn how to work with zeros using powers (e.g.: 4000000 = 4*10ˆ6)

Use math tools (Mimir math for iOS), Math tool on Viktor Cheng website to practice

3) Working with tables and graphs and deriving conclusions

  • Study "Say it with Charts" book
  • Check all available MBB presentations and publications. Practice to derive conclusions and check yourself with the actual ones from the article / presentation
  • GMAT IR part (Official guide and Manhattan prep)
  • Learn basic statistics (Any GMAT or MBA prep guides)

Good luck!

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replied on Sep 11, 2019
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Hi Sabrina,

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

  1. Repeat the question – candidates sometimes do mistakes answering the wrong question in the math part
  2. Present how you would like to proceed from a theoretical point of view (you may ask for time before presenting if you initially don't know how to approach the problem)
  3. Ask for time and perform the first computations
  4. Present the interviewer interim steps to keep him/her aligned – don’t just say the final number
  5. Continue with the computations until you find the final answer
  6. Propose next steps on the basis of the results you found

In terms of general math tips, 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, for instance, you could transform the computation in 320*10^7/73*10^7, making the overall computation faster.
  3. Keep good notes. One of the reasons people do mistakes with big numbers is that they don't keep their notes in order, thus forget/misreport numbers
  4. Divide complex math in smaller logical steps. This is something you can use for big numbers after the application of the 10^ power mentioned above. If you have to compute (96*39)*10^6, you can divide the first element in 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 art fractions and thus speed up/simplify the computation - the most useful to know are 1/6, 1/7, 1/8, 1/9.

Finally, in terms of formulas, this is what I would recommend:

  1. Write down all the new formulas/concepts you find when practicing cases
  2. Learn the formulas
  3. Look for quant parts in casebooks searching for the keywords interesting for you (eg DCF) to see possible applications for the most challenging ones

I would also recommend to practice math under pressure - not just math. Many candidates are totally fine with calculating 67% of 67 in a quiet environment, 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 environment of the interview.

Hope this helps,


replied on Sep 10, 2019
McKinsey Business Analyst | 3+ years Experience | Extensive experience in case preparation
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Hi Sabrina,

I don't know about other firms, I only prepared for MBBs.

My background is Mechanical Engineering, therefore you can bet I didn't have any clue whatsoever on what NPV was.

I wouldn't even bother learning it. MBB interviews are about how you reason, how your mind works (+ a FEW frameworks and key numbers), not about the formulas you know. Thus basic math ok, you could also be asked the perimeter or a circle or the surface of a box, but no super-complicated formulas.

To get better at mental calculations, there is no easy way. You have to practice, try to invent math calculations and solve them in your mind. That's the only way, unfortunately :)

However, if you struggle with quant problems in general, maybe there is a chance you haven't understand them completely. In that case, I would suggest talking to someone (coach, friends, peer, ...) and try to understand them together.