How to improve my mental math during cases

Mental Maths
New answer on Jan 04, 2023
7 Answers
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Anonymous A asked on Jan 02, 2023

Hi everyone, 

I have decent qualitative skills and business acumen. However, as soon as I get to the math part of the case I get lost and I take a lot of tome to calculate. Can you please recommend any solutions to get better in math? Where an I learn tricks, shortcuts, and rules to get faster and more accurate? thank you! 

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Francesco
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replied on Jan 03, 2023
#1 Coach for Sessions (4.000+) | 1.500+ 5-Star Reviews | Proven Success (➡ InterviewOffers.com) | Ex BCG | 10Y+ Coaching

Hi there,

Q: As soon as I get to the math part of the case I get lost and I take a lot of time to calculate. Can you please recommend any solutions to get better in math? 

It is difficult to answer without a concrete example of when you get lost. Please also note that most of the time you won't need to perform mental math and will be able to write down the numbers. I reported some general tips below.

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In terms of how to approach math during 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 math until you find the final answer
  5. Propose next steps on the basis of the results you found

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

  1. Use correctly power of 10. For example, 3.2B / 723M can be written as 3200*10^6 / 732*10^6
  2. Ask if it is fine to approximate. You can ask the interviewer if you can approximate complex math. If allowed, this will help to solve simpler problems. In the previous example, you could get 320*10^7 / 70*10^7
  3. Keep good notes. This helps to avoid to forget/misreport numbers
  4. Divide complex math into multiple simpler steps. For example: (96*39)*10^6 → 96*40 - 96*1 = 100*40 - 4*40 - 96*1 = 4000 – 160 – 100 + 4 → 3744*10^6
  5. Learn main fractions results. You can learn by heart fractions and speed up/simplify the computation - the most useful to know are 1/6 ~ 17%, 1/7 ~ 14%, 1/8 = 12.5%, 1/9 ~ 11%.

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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 asked this suddenly in a case interview.

In order to do so, try always to use a timer with a 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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Hagen
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updated an answer on Jan 03, 2023
#1 Bain coach | >95% success rate | interviewer for 8+ years | mentor and coach for 7+ years

Hi there,

I think this is an interesting question that may be relevant for many people. I would be happy to share my thoughts on it:

  • One important first step in improving your math skills is to get a handle on any anxiety or nervousness you may feel when working on math problems in any case study. This can be a common barrier to success in math, and it's important to address it before you can effectively work on improving your skills.
  • Once you have a plan in place for managing your anxiety, there are several strategies you can try to improve your math skills:
    • Practice, practice, practice: The more you work on math problems, the more comfortable you will become with the material. Consider setting aside dedicated time each day to work on math problems.
    • Review math concepts regularly: Make sure you fully understand the concepts being taught. If you are struggling with a particular concept, don't be afraid to ask for help or seek out additional resources to help you better understand the material.
    • Use online resources: There are many online resources, such as videos and tutorials, that can help you learn math concepts and techniques. These resources can be particularly helpful for visual learners.
    • Try using mnemonic devices: Mnemonic devices can be helpful for remembering formulas and concepts. For example, you might create a rhyme or acronym to help you remember the steps of a particular process.
    • Work with a coach: If you are still struggling with math, working with a coach can be a helpful way to get one-on-one assistance and personalized feedback.

If you would like a more detailed discussion on how to address your specific situation, please don't hesitate to contact me directly.

Best,

Hagen

(edited)

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Ian
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replied on Jan 03, 2023
#1 BCG coach | MBB | Tier 2 | Digital, Tech, Platinion | 100% personal success rate (8/8) | 95% candidate success rate
  1. 100% Recommend Rocket Blocks. It's exactly what you're looking for
  2. Online "Drills": https://www.preplounge.com/en/mental-math
  3. Math sheets (print these and do them on paper): Google Math Drills sheets
  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

Conceptual/Contextual Math

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

Key Math Equations

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

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Moritz
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updated an answer on Jan 03, 2023
ex-McKinsey EM & Interviewer | 7/8 offer rate for 4+ sessions | 90min sessions with FREE exercises & videos

Hi there,

You're potentially looking at it too narrowly. There's generally three few steps when it comes to quant and you're hyper focused on the 3rd:

  1. Knowing when to do quant and spotting the opportunities: Most candidates have no clue or are actively/subconsciously avoiding this. Even when the opportunity is staring them in the face, they will ramble about individual figures in data tables when they could just go ahead and do some meaningful quant to create real insights. This is very frustrating for interviewers and a key skill you must learn.
  2. Figuring out a target and setting up an equation to get you there: Many candidates just start calculating random things, hoping that it's somehow meaningful. This is bad an happens all too often. Instead, you must learn to determine a reasonable target, given what you have and can guesstimate where gaps exist, and develop and articulate an equation that get you there. This is not easy but can be learnt.
  3. Doing the actual math: This is what people are typically hyper-focused on, such as yourself. As a result, they often stumble over the previous points and don't even get a chance to letting their arithmetic skills shine.
  4. Developing insights: This is drifting off from quantitative to conceptual skills but still, it's important to contextualize any quantitative result properly. It hurts to see candidates succeeding in steps 1-3 and then failing by not really providing the “so what".

Given the above, what do you think is your real weakness? Let us know and we can guide you better!

Moritz

(edited)

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Anonymous updated the answer on Jan 02, 2023

I have a few tips that really helped me. Feel free to book a FREE intro call so we can discuss further.

There are three main things:

  1. Tricks such as left to right multiplication which will help you be fast and accurate.
  2. Drills focussing on multiplication and division of big numbers
  3. Identifying the type of case and where you can use shortcuts to solve the problem quicker (e.g. think before diving straight into the numbers)

(edited)

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Rushabh
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replied on Jan 03, 2023
Limited Availability | BCG Expert | Middle East Expert | 100+ Mocks Delivered | IESE & NYU MBA | Ex-KPMG Dxb Consultant

Hello,

I would strongly recommend for you to do the following:

1) Practice Drills - Use platforms like PrepLounge, CaseCoach and RocketBlocks. They have excellent self-paced drills where you can push yourself to get better at the specific area in math that you struggle with.

2) Peer Mocks - Whenever you do mocks with peers, make sure you tell them in advance to stress test you quant skills. Thus you can get some real practice with others.

3) Get a Coach - If you work with a coach, they will help you pinpoint the exact weakness you have in your quant, e.g. it could be that your mental math is great, but you fail to communicate the steps; or it could be that you take too much time because you write down too many steps and are poor at mental math! It's essential to practice with experienced folks to take you from a 6/10 to a 10/10! 

Hope this helps!

Feel free to message me directly via chat if you have any questions or would like to work with me :)

Best,

Rushabh

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Clara
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replied on Jan 04, 2023
McKinsey | Awarded professor at Master in Management @ IE | MBA at MIT |+180 students coached | Integrated FIT Guide aut

Hello!

Practicing with as many real cases as you can is the best way, for sure. 

A way to double down on that skill is leveraging math tools (Mimir math for iOS), Math tool on Viktor Cheng website to practice

Hope it helps!

Cheers, 

Clara

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Francesco gave the best answer

Francesco

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