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Times Tables 20-99

mental math
Recent activity on Jun 18, 2018
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Anonymous A asked on Jun 16, 2018

Hey,

On the preplounge math tools -some multiplications are between 20 and 99.

EG 24 x 45 / 63 x 18 / 91 x 22

How do you do these in your head fast?

cheers!

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Sidi
Expert
replied on Jun 18, 2018
McKinsey Senior EM & BCG Consultant | Interviewer at McK & BCG for 7 years | Coached 300+ candidates secure MBB offers

Hi Anonymous,

my advice here: do NOT do such calculations in your head during an interview! You will not get ANY bonus points for this! The only two things that count are

(1) You are consistently correct in your math

(2) You can be easily followed

For this, do WRITTEN calculations. When properly done, it barely takes more time. But it is a much clearer way of communicating - and this is what is much much more important than speed! Doing math in your head is absolutely not client friendly: the interviewer will assume that this is how you will also act with clients. If the client can not follow you (because you are doing math in your head and are not being explicit with it, it is your fault!

That being said - it is certainly not harmful to practice such multiplications tables. But in the interview, please do the math properly in WRITTEN form!

Cheer, Sidi

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Anonymous replied on Jun 16, 2018

How I would do the examples you provided:

24*45 = 10*45*2 + 4*45 = 900+180=1080

63*18 = 63*20-(63*2)=1260-126=1,134

91*22=90*22 +1*22 =90*20+90*2 +22 = 1800+180+22=2,002

In general, I try break up numbers into smaller segments that are easier to manage. It also depends if it's easier to subtract (as I did in the second example above) or just add up the various segments (as I did in example 1).

In general, just a lot of practice with this will get you a lot better and enable you to solve questions like this in your head in a short amount of time - but I would still write it out in an interview to be 100% sure!

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Vlad
Expert
Content Creator
updated an answer on Jun 16, 2018
McKinsey / Accenture Alum / Got all BIG3 offers / Harvard Business School

Hi,

I use the following method and recommend it to all of my students:

https://www.youtube.com/watch?v=6ndkkPZYJHo

You can learn to do it on paper fast in 1-2 hours, and it will take couple of days to train doing it mentally.

Other useful tools:

1) Learn how to work with zeros. Best way - always use 10^power instead of zeros

Example:

300x9000 = 3*10ˆ2 x 9*10ˆ3=3x9*10ˆ(2+3)=27*10ˆ5

Handwritten it looks not that complicated. If you get used to writing all the numbers that way, you will never loose zeros and all multiplications/divisions will be replaced with + or -.

2) Learn the division table up to 1/11 (i.e. 5/6 = 83.3%). It will help you calculate any percentage problems

Best

(edited)

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Anonymous B replied on Jun 16, 2018
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Sidi gave the best answer

Sidi

McKinsey Senior EM & BCG Consultant | Interviewer at McK & BCG for 7 years | Coached 300+ candidates secure MBB offers
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