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Mental Math Help!

Case Timing fast math mental math Mental Maths
Recent activity on Sep 24, 2020
4 Answers
1.9 k Views
Anonymous A asked on Sep 23, 2020

Hi everyone,

I just came off a practice case with an MBB senior manager - their biggest feedback for me is that I need to do my math "faster" (I took around 45 mins to do a 30 min case because I spent too long talking my thoughts out loud). Not sure about anyone else, but when I increase my speed I also increase the errors I make, and often descend into an anxious mess after I've messed up. Does anyone have concrete suggestions for how to improve this? I know can perform the maths well when not under pressure, but I don't want to get penalized for taking it slow.

Anyway, I'll obviously continue to practice, but would love some suggestions on the most efficient way to do this as my "real" interview is in 10 days!

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Best answer
replied on Sep 24, 2020
Ex-McKinsey final round interviewer | Executive Coach

When I was a candidate myself, I made a stupid math mistake (double counting) for my final round interview with a senior partner. Fortunately, it didn't lead to a rejection but I was asked to come back for an additional interview the following week where it was clear that the only expectation was for me to prove that I can do the maths correctly. I was a engineering student who had always been comfortable with numbers and so I knew a large part of it was bad luck and nerves as well.

It's hard to give generic advice but below are three things I did which I can reiterate having been on the other side of the table now.

1. Become comfortable 'running the numbers': I did the maths section of every case I could get my hands and practiced abstrat mental maths (e.g., long division). But most importantly, I started to force myself to see the bigger picture/dynamics behind the numbers than the explicit arithmetics (sometimes it's easy to focus purely on the digits when given large numbers and abstract percentages)

2. Develop your own communication style: depending on your comfort with numbers and under pressure, it's worth coming up with a style that works best for you. For some people, running the numbers independently then talking it through with the interviewer works much better than doing the maths as your interviewer watches. Additionally, often it's not neccessary to talk through and show every step of your thinking and there are logical shortcuts/rounding that can make a huge difference

3. Focus on nailing the 'so-what': the assessment is on your quantitative thinking NOT mental math. Drawing the implication or the 'so-what' from the maths is what differentiates you. Yes, that is contingent on you getting the maths right, but hopefully it takes off some pressure too...

Good luck and feel free to reach out if any questions!

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Content Creator
replied on Sep 24, 2020
#1 Coach for Sessions (4.500+) | 1.500+ 5-Star Reviews | Proven Success (➡ | Ex BCG | 10Y+ Coaching

Hi there,

To improve your performance you have to practice more math. Besides the resources mentioned by André and Ian, you can also skim casebooks and do the math part present there with a time constraint.

In general terms I would recommend the following to approach a math problem:

  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 interim steps to the interviewer 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

Hope this helps,


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Anonymous replied on Sep 23, 2020

Hi A,

What you need to do is literally practice more.

Practice math under limited time conditions while you have time before the interview.

Practice the interview situation as often as possible with your peers.

You don´t need to learn them by heart, rather skipping through to get a first impression and a better understanding.

If you need any further help or career advice, feel free to reach me out.



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Content Creator
replied on Sep 23, 2020
#1 BCG coach | MBB | Tier 2 | Digital, Tech, Platinion | 100% personal success rate (8/8) | 95% candidate success rate

Hi there,

Practice math every day!

Leverage the free preplounge resource:

Rocketblocks is great for math in the context of a case:

Print out math sheets for rote practide:

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Ken gave the best answer


Ex-McKinsey final round interviewer | Executive Coach
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