Sep 05, 2023

I have incoming interview and need to improve my mental math skills, ie learn tricks. Give me sources to study

Not sure why you got MS, it's a valid question and a big one in Consulting.

I have a few questions:
(1) when you say mental math, do you mean with a pen and paper (no calculator) or just purely in your head?
(2) What level of complexity are you aiming to reach? I think most folks can agree doing multiplication with decimals or four digits in their head is... not easy/possible.

Within Consulting, math drills are commonly asked or expected, I've personally used (and continue to promote) the Math Drills section of the Rocketblocks web platform. It provides a guided way to do multiplication and division with large numbers and gives drills for all the basic sets of arithmetic mathematics. It has built in features to help you test for accuracy and speed.

I would 100% recommend Rocketblocks, I've used it in the past and have continued (for 30 minutes a day / every few days for 2-3 weeks) each time I receive a case interview.

It was specifically built for interviews at MBB and other consulting firms, so the advice and drills are representative of the type of questions you may be asked in the case interviews.

I've always found it to be much easier when you're "big picture" regarding mental math (in your head). Round up or down to easier numbers (93 becomes 100, 104 becomes 100) and break large numbers down into smaller chunks 1508 x 93 becomes 1500 x 100 becomes (15 x 100  ) + (100 x 100). You could even break that last part down to 10 (10x100). For me it's just conceptually easier to break them down into smaller parts and add them. You'll get a general answer (because of the rounding) which is good enough for broad brush analyzing  large concepts in consulting (like how many washer / dryer units are purchased each yr in NYC). You look at the population. You gauge how much new construction. You guess how much replacement for old units based on average lifecycle, yada yada yada. The math part is just adding and multiplying.

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