Division with decimal numbers

math
New answer on Feb 02, 2020
5 Answers
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Anonymous A asked on Feb 01, 2020

Hi everyone!
What would be the simplest and most effective way to calculate division when divisor is a decimal number? For example, how would you do 340/0.55?

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Clara
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replied on Feb 02, 2020
McKinsey | Awarded professor at Master in Management @ IE | MBA at MIT |+180 students coached | Integrated FIT Guide aut

Hello!

Bear in mind that approximations are not only "accepted", but also a good sign (e.g., I prefer a candidate who gives me the "good big numbers" in a very agile way that one that takes long to give me the exact number)

In this case:

  • 0.55 can be rounded to 0.5
  • Dividing by 0.5 is the same as multiplying x 2
  • 340 x 2 can be broken down into:
    • 300x2 = 600
    • 40 x 2 = 80
    • 600 + 80 = 680
  • At this point, you can even realize that, since you divided by 0.5 instead of 0.55, your number should be smaller (by approx 10%, so you can approximate to 620 for simplicity of further calculations -the accurate number would be 618.18-.

Hope it helps!

Cheers,

Clara

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Antonello
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replied on Feb 01, 2020
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340 / 0.55 = 340 * 100 / 55 = 340 * 20 / 11 ≃ 20 * 341 / 11 = 20 * 31 = 620

For further math tricks I recommend the YT channel of this video: https://www.youtube.com/watch?v=PYrgjMubh-c

Hope it helps,
Antonello

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Anonymous on Feb 01, 2020
Great channel, thanks!
Francesco
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replied on Feb 02, 2020
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Hi Anonymous,

the most useful thing to do to improve accuracy in the math part is to increase the number of steps. That sounds counterintuitive but helps to simplify the computation and ensures you don't do mistakes. As an example:

  • 340/0.55 = (340/55)*100
  • = (34/5.5)*100
  • ≃ 6.2*100 = 620

You may calculate the exact value in the last step if needed.

Best,

Francesco

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Daniel
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replied on Feb 01, 2020
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here is a very clear tutorial on division with decimals https://www.youtube.com/watch?v=HlEx1TN-dqY

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Vlad
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replied on Feb 02, 2020
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Hi,

It's similar to 3/5 which is 0.6

Now, you just have to move the zeroes, and increase it a bit since the numerator is bigger. Give you 61-62

Some advice on fast calculations:

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!

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Clara gave the best answer

Clara

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