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Case interview - 6 common math mistakes and how to prevent them

Math in recruiting interviews


Strong analytical and logical skills are a prerequisite for basically every consulting companies, therefore showing strong quantitative skills is a prerequisite to get any consulting job offer.

In this article, I’ll go through the 6 most common math errors happening during consulting interviews and I will provide some suggestions on how to avoid them.

During my time as a recruiter for McKinsey I’ve been so often surprised in seeing how badly some of the candidates prepare to do math. So guys, here is the list of the most common math mistakes you can do during consulting interviews:


All interview cases to different extent include some math: usually very elementary calculations, some other times it can get a bit more trickier but in any case it will always be limited to the four basic operations (addition, subtraction, multiplication and division).

A couple of times I’ve been asked by the candidate if he/she could use a calculator... well, in case you are still wondering, the answer was and still is no!

Being able to quickly do mental calculation is important not only during interviews but sometimes also during real project cases, when I have to assess on the spot during client discussion the potential quantitative implications of some decisions and give my opinion about them.

Based on my experience the math part that gives candidates the most trouble is the use of percentages.
How to prevent the mistake:

Practice! As stated before, you will never be asked to use derivatives or integrals but all you are required to know are the four basic operations. You have no excuses to not practice. Your math skill is like a muscle: you have to exercise it if you don’t want it to waste away.

Make sure you understand and can use percentages, ensure that you can calculate comfortably 18% of 120 or you can multiply two-digits numbers. This will be enough to solve 100% of the cases. Just practice!


This is typical! When a candidate screw up an easy calculation he will start to get agitated. When the next calculation comes up, he will try to make up for his earlier mistake, get anxious and make another mistake.

How to prevent the mistake:

In the event that you make an error, relax; One mistake will not get you dinged, just ask for a minute to go over your work, find the mistake and correct yourself. Even if you miss the chance to correct yourself, shake it off, and move on to the next part of the case. Remember that at any time you can ask for a minute to gather your thought and do a calculation. Asking for some time is never perceived as a bad signal but rather as a sign that you are in control of yourself and of your time.


Doing math in your head is definitely not an error, but always remember to guide the interviewer step by step through how you are trying to approach the problem. Just consider that making a mistake in your head without voicing the steps you took can be a problem for several reasons:

  • Firstly, you cannot go back and find your mistake because there isn’t any track of your calculation

  • Secondly, the interviewer is not able to help you by letting you know where you went off track

  • Thirdly, the interviewer cannot give you credit for the share of the calculation you did correctly, because you never showed him.

How to prevent the mistake:

Voice your approach and guide the interviewer step by step! No one is ever going to congratulate you because you did some calculations in your head but rather the interviewer might think you just got lucky with your number.

I would strongly recommend to explain and write each and every step of the calculation so that the interviewer can follow along and help you if necessary.


Remembering the units of the numbers you are using in your calculation is extremely important for several reasons.

  • First, it clearly shows the interviewer that you are not simply doing some math but you understand the context of the calculation within the business case

  • Second, the units make the answer meaningful. If you are asked what’s the target profitability of a retail chain and you answer “10 millions”, my natural reaction would be “what does this number mean? Are we talking about 10 millions dollars? Euros? Is it per year? Per month? Clearly the units associated with the 10 millions are important

  • Third, performing calculation with units can be of great help to understand early on if you're making an error. If you find yourself multiplying $ by miles per hour, chances are you've made a mistake somewhere along the calculation.

How to prevent the mistake:
Always make sure to label each step of your calculation and remember to carry the units as you work with them. Keeping your calculations well organized will prevent you from losing track of the units. When you complete your math, show your final number with the appropriate units.


The math performed during a consulting case interview is never there just for the sake of doing math; conversely the calculations are part of a bigger business problem you have to solve. Based on my experience though many candidates get wrapped up in calculations, arrive to the correct final number but forget why they were doing the math and the real purpose of the number they’ve just found out.

In one of my cases I like to ask candidates to determine the expected 2020 profitability of an industrial company, to reach this result all the costs components are given while the candidate only have to do some math to estimate the company revenues. I’ve seen many interviewees doing properly all the math but at the same time completely losing sight of what they have just calculated and telling me that the results they got was the 2020 profit (while unfortunately they calculated just the revenues and therefore only the first step to reach the final company profitability). In short many candidates lose sight of the end goal and confuse the result of their calculation with the final answer to the case question.

How to prevent the mistake:

The secret here is simply to remember the significance of the figures you are calculating in the context of your particular business case. One tactic I like to see from the candidate is to write the question asked at the top of your sheet before deep diving in your math. As you go through your calculation and as you prepare to present your solution, keep on reminding yourself the question you were originally asked and ask yourself if the result you got from your calculation is actually answering it. Then when you explain your answer, do so in a way that clearly shows you understand what your final number means, for example you could explain the implications of the answer on the business case.


As already mentioned math mistakes are completely acceptable, what instead is not tolerable is not being able to catch your mistake even though the answer you got makes absolute no sense in the context of the case. If that happens you have added poor judgment to your miscalculation.

One recent example of this mistake comes from a candidate who told me that according to his calculation the average salary of a blue-collar in Germany was close to 10 M$ per year. Clearly this candidate not only screwed up his calculation (that is acceptable) but also completely lacked common sense in his answer (that is much worse).

How to prevent the mistake:

Every time you reach the end of your calculation do a reality check, ask yourself if your number does make sense in the context of the case. For example remind to ask yourself questions like: is this the right order of magnitude given the other figures of the case? Shall I expect the number to be positive or negative? If I am considering a ratio, should it be greater or less than one?

Ok guys, if I was not able to answer all your questions about this topic or you would like me to touch any additional issues, feel free to leave a message on this page or through the contact form.

I will read and reply to your messages.



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