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# Hi coaches, could you plz provide some advise for improving mental calculations?

calculation math
Recent activity on Aug 22, 2020
1.7 k Views

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Hello!

The best is to practice, practice, practice! I could recommend you to start with:

1. GMAT exams: that can be found for free in the internet (GMAT prep with The Economist, London Business School free trial, etc.)

2. Math tools: such as Mimir math for iOS or Math tool on Viktor Cheng´s website)

3. Multiplying any two 2 digit number in 3 seconds: check this great video > https://www.youtube.com/watch?v=6ndkkPZYJHo

Best,

Clara

Hi,

First of all, I recommend using the following approach while doing your calculations

1. Tell the interviewer your approach / formula

2. Check with the interviewer that your approach is correct

3. Ask for a minute to perform the calculations

4. Check any assumptions you make with the interviewer. If you need to round the number - you have to check with the interviewer as well

5. Come to the interviewer with some intermediate calculations results to check that you are moving in the right direction

6. Come up with the final answer

7. Provide the conclusions from the final answer

The following skills will speed up your calculations and reduce the probability of making a mistake:

1) Learn how to solve equations and systems of equations. As you could have noticed, some cases require solving an equation. You can use GMAT books to practice.

2) Learn how to multiply double-digit numbers really fast. It takes just a couple of hours to learn how to apply this method on paper and a couple of days to start doing these calculations mentally, but it's worth it. Please follow the link for more details: https://www.youtube.com/watch?v=6ndkkPZYJHo

3) Learn how to work with zeros.

Simply always use the powers of 10. Then you'll be able to multiply / divide the numbers and sum up / subtract the powers separately:

For example: 300 x 9000 = 3 x 10ˆ2 x 9 x 10ˆ3 = 3 x 9 x 10ˆ(2+3) = 27 x 10ˆ5 or 2.7 Mln

If you get used to writing all the numbers that way, you will never lose zeros.

4) Learn the division table. This method will help you calculate any percentage problems like market shares or margins. For example, if your market is \$620M and your revenues are \$5.1M you can use 5/6 or 83.3%, as a proxy to calculate the market share. By adjusting to zeroes and slightly decreasing the number, you'll get 8.2%

Best!

Dear Isabel!

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). But there are some facts that will help in overall improvements:

Consulting math is very different than academic math. Working consultants - and consulting interview candidates - are always under time pressure. Results are what matter and answers are required simply to be good enough to guide business decisions, rather than being absolutely correct.

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 question. Interviewers notice this, and this will not give credit.

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

Best,

André

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McKinsey | Awarded professor at Master in Management @ IE | MBA at MIT |+180 students coached | Integrated FIT Guide aut
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