Math mistakes in interviews

case math Final Round McKinsey mistakes
Recent activity on May 23, 2019
4 Answers
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asked on Dec 09, 2018


I have had one of my final interviews yesterday with McK. After the interview, I started to suspect whether I had done a math error in a calculation. However, the interview went pretty smoothly and my interviewer did not point out to a mistake and he did not challenge my conclusion. If a candidate does a math mistake, do interviewers point out to it?

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replied on Dec 09, 2018
Ex-MBB, Experienced Hire; I will teach you not only the how, but also the why of case interviews

In my experience, interviewers will and do point out math mistakes - I know I do. Yes it is stressful, but how you respond is telling us a lot. It is often ok to make ONE math error btw, I probably did one in every single one of my interviews, ever. What is NOT ok is to make two math mistakes.

Based on this, I would venture you did not make a mistake in the interviews - and if you did, it may not have been very important so the interviewer elected to let you fly with it.

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replied on Dec 09, 2018
McKinsey / Accenture Alum / Got all BIG3 offers / Harvard Business School


It may be both. Or maybe he hasn't noticed the mistake himself. So just relax and keep fingers crossed.


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Arshad replied on Dec 09, 2018

Good day

In my experience as a recruiter for the past 10 years, I generally do not point out the mistakes of candidates in the interview to avoid contributing to what already may be a stressful situation for some.

The purpose of the interviews is an information gathering exercise for both parties and evaluation for the job.

The feedback is usually not instant with interviews.

Hope this helps.

Kind regards

Arshad Sheik

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replied on May 23, 2019
Ex M&A banker and Consultant at JP Morgan and Oliver Wyman

During the interviews that I gave, I would highlight this. There is a relatively small chance than an interviewer does not see a math mistake (especially since they have run those numbers during hundreds of interviews). However, mistakes are also part of your normal job as a consultant the key thing is whether the interviewee can spot his own mistakes, however if he didn't how does he continue after a mistake (does he spot similar mistakes in the rest of the assignment / does he get nervous etc)

What I would recommend is the following during math questions:

  • Think out loud and show the math steps that you are taking
  • Use short-cuts when writing down your numbers. I see many interviewees write 2,000,000 * 140 = 280,000,000. This is confusing for your self and takes a long time
    • Instead try: 2*10^6 * 14*10^1, which simplifies the calcs you need to do: (A) 2*14= 28; (B) 6+1= 10^7; So COMBINED: 28 * 10^7
  • Lastly, always do a sanity check
    • If you have a cost of 700k per year with a revenue of 100mm, always sanity check if you haven't made a mistake. Those kind of differences indicate that you might have made a mistake with a 0. Spotting such an error during your interview gives great confidence to the interviewer that you would be able to spot your mistakes rather than blindly presenting figures to your manager or client
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Guennael gave the best answer


Ex-MBB, Experienced Hire; I will teach you not only the how, but also the why of case interviews
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