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Help with a mental math question

case math Math problem
New answer on Mar 21, 2023
6 Answers
486 Views
Joël asked on Mar 20, 2023

Hi, I'm struggling with a simple mental math problem

Please could you share how you would go about calculating 80m/9,040 without using a calculator? 

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Best answer
Cristian
Expert
Content Creator
replied on Mar 20, 2023
#1 rated MBB & McKinsey Coach

Hi there, 

The trick is to break down each number into common denominators:

80 / 9,040 

=

8 * 10 / 904 * 10

=

8 / 904

=

2 * 4 / 2 * 452

=

4 / 452

=

2 * 2 / 2 * 226

=

2 / 226

Then you have to make the actual division

= 0,0088.

There are lots of websites where you can practice fast maths - check out online.

Hope this helps!
Cristian

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Ian
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updated an answer on Mar 21, 2023
#1 BCG coach | MBB | Tier 2 | Digital, Tech, Platinion | 100% personal success rate (8/8) | 95% candidate success rate

Hi Joel,

Make sure you're learning all of the techniques for fast math. There are so many resources online!

For 80m/9,040 I would:

Round to 80,000,000/9,000

Cut the zeros to 80000/9

Round 9  to 10 so 80000/10

Recognize that while this # is 8,000, I rounded 10% up on the deonominator so a smaller demonimator would be somewhere between 8,500 and 9,000….I'll go with 8,750

(edited)

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Hagen
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Content Creator
replied on Mar 21, 2023
#1 Bain coach | >95% success rate | interviewer for 8+ years | mentor and coach for 7+ years

Hi there,

I think this is an interesting question that may be relevant for many people. I would be happy to share my thoughts on it:

  • First of all, I would advise you to divide both numbers by 80, given that 9,040 is a multiple of 80. This would result in 1m / 113 = 1,000k / 113.
  • Next, you should think about how often 113 fits inside 1,000, which is almost 9 times. As such, the result will be very close to 9,000 (i.e., 8,849).
  • Still, while this might be a calculation that you are expected to perform mentally, it is absolutely fine to perform most calculations in writing.

If you would like a more detailed discussion on how to address your specific situation, please don't hesitate to contact me directly.

Best,

Hagen

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Paul
Expert
replied on Mar 20, 2023
PL-level BCG experience (6 years)|Interviewer at BCG| 6/6 personal + 95%+ candidates offer success rate

Hi there,

what I would do:

1) I would ask to the interviewer if possible to approximate to 80M/9k (you are approximating denominator by 0.5% - should not be an issue)

2) Symplifying zeros: 80M/9k = 80 000 000 / 9 000 = 80 000 / 9 

3) Using fractions to further simplify = 80000 / 9 = 1/9 * 8 * 10000 = 0.1111 (you should learn simple fractions “by heart” ½= 0.5 ⅓,= 0.33, ¼ = 0.25, , 1/5, 1/6, 1/7, 1/8, 1/9) * 8 * 10000 = 8888.9

Hope this helps

Feel free to DM for more details

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Pedro
Expert
replied on Mar 21, 2023
Bain | Roland Berger | EY-Parthenon | Mentoring Approach | 30% off first 10 sessions in May| Market Sizing | DARDEN MBA

80m/9040 = ~90m/9k - 10m/9k = 10k - 1.1k = 8.9 k

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Francesco
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replied on Mar 21, 2023
#1 Coach for Sessions (4.500+) | 1.500+ 5-Star Reviews | Proven Success (➡ interviewoffers.com) | Ex BCG | 10Y+ Coaching

Hi Joël,

Q: How you would go about calculating 80m/9,040 without using a calculator? 

I would do the following:

  1. Ask if you can approximate
  2. If that’s possible, simplify it as 80m/9k = 80k/9 ~ 9k
  3. Bonus: since the denominator is a bit higher, explain the real final number should be slightly lower

If they don’t want you to approximate, you will have to do the actual math as 8M/904. A possible way to do so is what shown by Cristian.

Best,
Francesco

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Cristian gave the best answer

Cristian

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