How fast should 66*38 be calculated?

mental math
New answer on Dec 07, 2019
4 Answers
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Nils asked on Dec 02, 2019

Hi,

Should I learn to calculate this (66*38) in my head, or is it okay to write

66*38 (40 - 2) = 2400+240 = 2640-132 = 2508.

Writing it down takes me 30 seconds. I guess it has to be reduced, but how quickly do you think that this should be solved?

Any help would be appreciated! :)

/Nils

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Antonello
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updated an answer on Dec 02, 2019
McKinsey | MBA professor for consulting interviews

Hi Nils,
the approach is correct and I think you can reach half of the time with it by practicing a bit. Preplounge math tool is super helpful and I also love this Youtube channel (here a video for fast multiplications): https://www.youtube.com/watch?v=PYrgjMubh-c

Hope it helps,
Antonello

(edited)

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Nils on Dec 02, 2019
Great, I think I can reduce the time by practicing. But is it okay to write out the calculation by hand?
Antonello on Dec 02, 2019
of course not necessary (mental the better), but if needed it is fine. In this case I recommend to use another sheet (to leave clean your main page with the structure)
Nils on Dec 03, 2019
Thank you! I have been recommended that too. I do the easy and medium math exercises in my head, but I struggle to compute the hard questions in the head as there are so many numbers to remember. Do you have a youtube movie that shows how to remember the numbers while calculating?
Vlad
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replied on Dec 03, 2019
McKinsey / Accenture Alum / Got all BIG3 offers / Harvard Business School

Hi,

You need to develop 3 calculation skills:

1) Learn how to multiply double-digit numbers (https://www.youtube.com/watch?v=6ndkkPZYJHo).

2) Learn how to work with zeros. The 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 lose zeros and all multiplications/divisions will be replaced with + or -.

3) Additionally, I suggest to learn how to make the division mentally:

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

Finally, use math tools (Mimir math for iOS, Math tool on Viktor Cheng website) to practice. Train, train, and train again

Good luck!

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Nils on Dec 03, 2019
Great advice! Antello also shared the same methodology for calculation. It seems like the cross multiplication method it is quite effective. There are many methods to calculate. Above, I did the "subtraction method", Kshitj recommended the "factoring method". Is it valuable to focus on one technique or multiple techniques for calculation? I am also eager to learn more about the division table. Do you have a link to that table?
Kshitij
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replied on Dec 03, 2019
12+ years in Strategy Consulting|EY|IBM|Inverto GmBH|Monitor|Accenture

Hi Nils

The more you practice the more ideas and appraoch you willl come across that would reduce the time you might be taking to calculate.

Herea are a few, but to master these one has to practice regularly

1. Simplify the numbers 66*38 = 2*3*11*2*19

Learn few tricks eg. multiplying my 11 it's the easiest ... 2*3*2*11 *19 = 209*3*4 = 627*4= 2508

2. Learn and practice vedic calculation techniques for two ot three digit numbers. These are very very simple and you would be able to marster these with few days practice.

Yes, Practicing by writing down the calculations will help you sharpen the visualization of numbers and also trigger fidentifying faster techniques.

Hope this helps. Incase you need any help with vedic calculation techniques please free to connect. I developed a pattern based technique for multiplication of large numbers during my engineering days that pretty easy to execute once practiced, can share with you its derivation and application.

All the best.

Kshitij

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Nils on Dec 03, 2019
Thank you! I am doing a lot of vedic division at the moment, but I think it would be very valuable to do more of it in the multiplication to do. Thank you! All of the recommendations has been around Vedic maths, so I guess that is the best practice! Thanks for your help! :)
Deniz
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replied on Dec 07, 2019
BCG | A.T. Kearney | University of Cambridge | 350+ coachees | 5+ years of consulting experience in London, Dubai and Istanbul

Hi,

It is also okay to write it down, as long as you get to the answer to this particular equation in 10-15 seconds or so. I would break 38 into 30 and 8 when doing the calculation.

Best,

Deniz

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Antonello

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