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Moltiplications/divisions with 6+ digits numbers, estimation math_Tricks

Anonymous A asked on Nov 12, 2018

Is there any way/trick to solve estimation math? I was reading the comments here that apply to 4 digits numbers, how about millions and billions?

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Francesco replied on Nov 12, 2018
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Hi Anonymous,

in general, the main issues with big numbers is forgetting zeros during operations. Potential tips to deal with big numbers are the following:

  1. Use correctly 10^ powers in your math computation. For example 3.2B/723M can be transformed in 3200*10^6/732*10^6, which makes it easier to deal with math
  2. Ask if it is fine to approximate. When you have to deal with math in market sizing, and sometimes even in business cases, you are allowed to approximate math to simplify the computation. In the previous example, for instance, you could transform the computation in 320*10^7/73*10^7, making the overall computation faster.
  3. Keep good notes. One of the reasons people do mistakes with big numbers is that they don't keep their notes in order, thus forget/misreport numbers
  4. Divide complex math in smaller logical steps. This is something you can use for big numbers after the application of the 10^ power mentioned above. If you have to compute (96*39)*10^6, you can divide the first element in 96*40 - 96*1 = 100*40 - 4*40 - 96*1 = 4000 – 160 – 100 + 4 = 3744*10^6
  5. Use shortcuts for fractions. You can learn by art fractions and thus speed up/simplify the computation - the most useful to know are 1/6, 1/7, 1/8, 1/9.

Hope this helps,
Francesco

Thank you very much Francesco for the reply. Do consulting firms expecting candidates to do this kind of calculations mentally? — Ornella on Nov 12, 2018

Hi Ornella, for big numbers with 6+ digits you will normally be able to write them down and do the computation on paper. Hope this helps, Francesco — Francesco on Nov 19, 2018

Vlad replied on Nov 13, 2018
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Hi,

I recommend using several tools:

  • Rounding up and down
  • Learn how to multiply double digit numbers (https://www.youtube.com/watch?v=6ndkkPZYJHo)
  • Learn the division table up to 1/11 (i.e. 5/6 = 83.3)
  • Learn how to work with zeros using powers (e.g.: 4000000 = 4*10ˆ6) and sum up / subtract the powers separately

Use math tools - Preplounge, (Mimir math for iOS), Math tool on Viktor Cheng website to practice

Best!

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