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Moltiplications/divisions with 6+ digits numbers, estimation math_Tricks

Case Maths estimation fast math GMAT MathSkills IB Guessestimation Math problem
New answer on Nov 16, 2020
4 Answers
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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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Content Creator
updated an answer on Jun 16, 2020
#1 Coach for Sessions (4.500+) | 1.500+ 5-Star Reviews | Proven Success (➡ | Ex BCG | 10Y+ Coaching

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 heart 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,


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Ornella on Nov 12, 2018

Thank you very much Francesco for the reply. Do consulting firms expecting candidates to do this kind of calculations mentally?

Francesco on Nov 19, 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

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


My 2 advises for this are -and I struggled with the same thing-:

  • Round up numbers
  • Use scientific notation to navigate the zeros

Hope it helps!



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Anonymous replied on Sep 21, 2020

Dear A,

On the top of what has been already mentioned I would like to recommend you for practicing and improving your mental math you can find also some apps and tools, besides Victor Cheng’s math tool (Magoosh's mental maths app, Mental math cards challenge app etc).

The next important thing is that rather make it on the paper to structure the notes and then to communicate the results clearly rather than make your calculations fast

The time pressure in case interviews is severe and you cannot afford to waste time. But to make your calculations right you shouldn’t be in a rush. So, I would recommend you here to work on both - practice with time limitations and learn how to keep your mind peaceful and concentrated (it might be not so popular advice here, but mediations really make their job here ).

Be comfortable and confident to state your answers not as a questions. Interviewers notice this, and this will not give a credit.

If you need any further help or career advice, feel free to reach me out.



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replied on Nov 13, 2018
McKinsey / Accenture Alum / Got all BIG3 offers / Harvard Business School


I recommend using several tools:

  • Rounding up and down
  • Learn how to multiply double digit numbers (
  • 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


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