Mental Math Fast! – Victor Cheng

Some things in life are NOT natural. Doing math computations in your head, while an interviewer is staring at you, with the prospect of a $100,00o or $225,000 job offer at stake and your entire career future at stake, is NOT a natural act.

I have received numerous emails from PhDs from MIT in Physics and Engineering who failed a case interview because they made a computational error. Obviously people with perfect GRE scores can do math taught to 11-year-old children, but doing it UNDER PRESSURE with another person in the room, staring at you, waiting impatiently for the answer, is an entirely different experience.

In a case interview, 95% of the time a single mental math error results in an automatic rejection. This is because firms like McKinsey, Bain and BCG can not take the risk that you will make a mental math error in front of a client that’s paying them $1 million a month for your and your colleagues’ time.

If you are even considering a career in consulting, I can guarantee you that you already know enough math concepts to pass the case interview. It’s called addition, subtraction, multiplication and division. In consulting, we also often use percentages. That’s 90% of the math we use in the case interview. Clearly, you already know how to do that.

What you may not know how to do, or have enough real world experience doing, is doing such computations in your head, using large numbers.

For example, what is the approximate answer to 500 million divided by 150 multiplied by 15%?

Go!

One second… two seconds… three seconds…

Do you know the answer?

If you haven’t figured out the correct answer by the time you read this sentence, your current mental math skills may be too slow or not accurate enough to pass a case interview.

Incidentally, the precise answer is 500,000, but any answer between 400,000 – 600,000 would have been acceptable by most interviewers.

Here’s the big secret to doing mental math fast (and accurately). You do NOT do mental math in the same way you do math using paper and pen, or with a calculator. The sequence of operations is totally different. In mental math, you actually take MORE steps than when you do math on paper or via calculator. Yes, you actually make the math problem LONGER mentally, but in the process you make the problem SIMPLER to solve. THAT is the counter-intuitive secret to doing mental math both faster and more accurately.

Because so many people struggle with the mental math aspects of the case interview, I’ve put together a very useful and immediately applicable video tutorial on how to do case interview math quickly and accurately while under pressure. In my Mental Math Fast! video tutorial, I cover:

✓ How to recognize when you don’t need to actually do computations to answer the qualitative question the interviewer is asking
✓ Different ways to simplify math problems that will increase computational speed and accuracy
✓ A simple strategy for being able to remember complex numbers in your head (after all, if you do the math correctly, but forget the actual values to use, you will get the wrong answer)
✓ How to tell when an approximation is “good enough,” versus when you need to perform the precise calculation
✓ How reversing the order of some calculations will often get you a faster answer

Mental Math – Fast! includes:
✓ A video tutorial showing you these mental math strategies and tricks, how and when to use them, and how to practice them in preparation for a case interview
✓ Transcripts from the video tutorial
✓ Presentation slides for easy reference
✓ Instant Recognition Math Reference Handout – a quick guide to figuring out which mental math approaches to use in specific situations