Hi there,

Q: What is the difference between a “forgivable” math mistake in an interview versus an “unforgivable” math mistake, and how to avoid them?

I don’t think there is a perfect line between the two. In general, if you make a minor mistake and/or you catch it, you could be fine.

Usually mistakes in math are due to the following:

1. You misunderstand the objective and answer the wrong question
2. You don’t ask the right clarifying questions
3. You are unable to ask for help when needed
4. You didn’t do enough drills, thus you have seen a limited amount of math problems and cannot quickly identify the optimal structure
5. You get confused by a large amount of information

In any case it doesn’t really matter, because your goal should be to do zero mistakes. So I would concentrate on the tactics to decrease the probability to zero as much as possible.

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In terms of how to approach math in 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 computations 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 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 you could transform the computation in 320*10^7/70*10^7
3. Keep good notes. One of the reasons people do mistakes with big numbers is that they don't keep their notes in order and forget/misreport numbers
4. Divide complex math into logical steps. This is something you can use for big numbers after the application of the 10^ power mentioned above. For example: (96*39)*10^6 à 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 heart fractions and thus speed up/simplify the computation - the most useful to know are 1/6, 1/7, 1/8, 1/9.

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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 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 conditions of the interview.

Hope this helps,

Francesco