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Mental math - division

New answer on Nov 16, 2020
4 Answers
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Anonymous A asked on Sep 16, 2019

Hi everyone,

I feel pretty confident in my mental math skills, but sometimes I need additional time with division. Could you give me some hints for examples below.

-2.3/3.6 - I would multiply by 10 - 23/36 ≈ 60%

-70/85% - no clue for a shortcut

-707k/ 18 910 - 7 * 10^5 / 1.89 * 10^4 -- I know it is between 3.5 and 4, but don't know the exact

-88/1400 -- ?

Would love to hear your mental processes, thank you!

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replied on Sep 17, 2019
McKinsey / Accenture Alum / Got all BIG3 offers / Harvard Business School


Simply learn the division table up to 1/11 (i.e. 5/6 = 83.3%). It will help you calculate any percentage problems. For example, if your market is $620M and your revenues are $51M you can use 5/6 as a proxy to calculate the market share


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Anonymous C replied on Nov 16, 2020

On top of learning table up to eleven, but actually you only need the table of 2,3,5,7 and 11 the rest is useless as you'll simply derive it from the other one, one thing that usually helps is to multiply.

One thing that i noticed is that people have usually less trouble to estimate fraction bigger than 1 than smaller than one. For instance they would have less trouble to estimate 12/7 than 6/7.

So my advice would be to multiply to obtain something more convenient.
Another advice is to split your division.

23/36? Ok that's 20/36 +3/36. The first sum is half of 40/36 so half of 36/36+4/36. So 1+0.1+0.01+.... So 1.1 to call it short. The second part is 1/12 so 8.5%.

So practice that gymnastic both for mental calculation and on paper. During an interview, if you do this quickly on paper while at the same time explaining your process, your interviewer will actually prefer it much more than having a hard time to follow you because you do a lot of fast calculation in your head.

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Content Creator
replied on Nov 16, 2020
McKinsey | Awarded professor at Master in Management @ IE | MBA at MIT |+180 students coached | Integrated FIT Guide aut


On top of the 11 table advise -which applies to many issues-, I wouldn´t stress to much, since you won´t get this complicated ones in an MBB interview.

Hope it helps!



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Anonymous B replied on Oct 21, 2019

There is just one thing: practice, practice, practice, ...

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