So, without using a calculator: what is 40 x 36? What is 12,5% of 160? What percentage is $ 2 million of $ 14 million? If we sell product A 5.000 times for $ 1,49 a piece, what is our revenue?

You should be prepared to answer questions like these during your case interviews, whether they are part of the case you’re solving or fired at you by the interviewer in an attempt to test your quantitative skills. All without the use of a calculator. So why is the interviewer looking for skills you probably have not used since elementary school and in a world with calculators in every mobile phone?

##### The importance of math

First of all, being able to calculate both fast and correctly in your head is of great value in the consulting world. You’ll deal with large sets of numbers on a daily basis and being able to calculate these numbers without cranking out a calculator every time just saves lots of time. It will also help you get a sense of the magnitude of these numbers:

I always want to know how an interviewee reacts to large numbers. Does he or she calculate them with ease? Does the candidate grasp the effects of a cost saving, for example? A cost saving of $ 0,56 per product might not look like much, but what if we produce 9,5 million each day? I want a future consultant to grasp the magnitude of these numbers, which is difficult at best if you need that calculator every time.

It helps finding mistakes in calculations, caused by a typo during a late-night spreadsheet marathon, for example. Is the total yearly market for running shoes in the US really only $ 2 million a year?

Secondly, being able to calculate quickly in your head looks good in front of the client and prevents embarrassing mistakes. The interviewer wants to know whether he can take you to that client presentation and rely on your performance if a client asks you what the effects would be if they increased prices by 15 %.

##### Getting your math skills up to par

So, you probably haven’t been calculating in your head since elementary school, but you do want that job in consulting: what to do? Unfortunately, there’s no easy way out: calculating in your head is a skill and the only way to improve it is by practicing it over and over again. You’ve probably ditched the old math books, but luckily there are plenty of great resources online. Below, we’ll go through some of them.

##### Kofprechnen Trainieren

A tool you can download and use offline, if you wish. Even though it’s in German, the software is pretty straightforward. A great way to practice that I’ve used myself and unlimited arithmetic fun!

##### SIKORE

A tool you can use both offline and online, doesn’t feature a time component, but if you’re on a Mac and can’t use Kopfrechnen, this is a great alternative.

##### Youtube: Math Tricks

This video series teaches you some handy math tricks and is worth a look. Changing the techniques you use for calculus might lead to confusion during the interview, so if you only have one week to prepare for the interview, I suggest sticking to your owntrusted methods. Secondly, this video links to a (paid) teaching method that I do not endorse.

[tube]https://www.youtube.com/watch?v=hLdKsKep1og [/tube]

Good luck with practicing!

– Stephan