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Math

mental math
New answer on Oct 28, 2021
5 Answers
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Anonymous A asked on Oct 24, 2021

Prepping for cases and trying to figure out how long is too long to do the math portions. For example, is it okay for a set of math equations, such as sets of 3 digit multiplications, to take a minute or two? Will I be dinged for taking a bit longer? I know it's a vague question but trying to determine the typical response times and what's expected of me!

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Ian
Expert
Content Creator
replied on Oct 25, 2021
#1 BCG coach | MBB | Tier 2 | Digital, Tech, Platinion | 100% personal success rate (8/8) | 95% candidate success rate

Hi there,

It's really hard to provide you a set rule here as it truly is contextual.

The general rule is that math should be reasonably fast and accurate. If the math is simple, it should take 10-20 seconds. If the math is intense, it could take 2-3 minutes.

I have a few thoughts here:

  1. You will not have a set of 3 digits times 3 digits multiplication (without rounding)
  2. Case other people and see how long they take - judge what you feel is too long/slow. Then self-assess. Use your own judgement here!
  3. There are plenty of math tools out there. Preplounge has a good one that helps you better understand speed needs: https://www.preplounge.com/en/mental-math
  4. Make sure that, if you have a lot of math, you're popping your head up at key intervals to inform them of intermediate findings/#s
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Pedro
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replied on Oct 25, 2021
30% off in March 2024 | Bain | EY-Parthenon | Roland Berger | Market Sizing | DARDEN MBA

To be honest, I don't think there are any complex 3 digit multiplications on real cases. 

For exampl, if you have to multiply the days of the year, the best way is to assume 350 days (multiply by 7, add 2 zeros, and then divide by 2). Same with any other 3 digit number, you ask to use a user-friendly number that is sufficiently close, and then use it for the estimation.

And no, you won't be “dinged” for taking a bit longer on a complex calculation, that is not what we are evaluating here, so don't fret about it (but practice 2 digit number multiplication).

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Anonymous A on Oct 25, 2021

Thank you! Can you please explain your example? I just want to make sure I understand the reference. If you already assume 350 days, what is the reason for the calculation you stated? Thanks so much!

Pedro on Oct 25, 2021

It should be quite straightforward to multiply by 7 and to divide by 2 (and then to add a couple of zeros). At least, much easier that multiplying by 350 (or 35 and adding a zero). This is how people that can do complicated math operate: they transform the calculation into another one that is easier to perform.

Antonello
Expert
Content Creator
replied on Oct 25, 2021
McKinsey | NASA | top 10 FT MBA professor for consulting interviews | 6+ years of coaching

Hi!

It's normal that more complex calculations take more time.

3-digit multiplications are not that common as you are often able to simplify the calculations (e.g. by rounding).

My advice is to go through fast math articles here on PrepLounge and get quicker in solving 2-digit multiplications.

Hope this helps.

Best,

Anto

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Clara
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replied on Oct 28, 2021
McKinsey | Awarded professor at Master in Management @ IE | MBA at MIT |+180 students coached | Integrated FIT Guide aut

Hello!

Agree with what has been outlined already: 

  • There are no long math calculations that would take you 1-2 per operation. If you see yourself in that situation, probably u are making it more complicated than needed 
  • There is not such thing as a standard timing, it should be fluid and agile

Hope it helps!

Cheers, 

Clara

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Agrim
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Content Creator
replied on Oct 27, 2021
BCG Dubai Project Leader | Learn to think like a Consultant | Free personalised prep plan | 6+ years in Consulting

3-digit by 3-digit multiplication should not take more than a minute under most circumstances.

If it is taking you longer than that - then I suggest to put in some math drills and speed-up your calculations.

Having said that, there are 2 things to ponder over:

  • Typically you do not get 3-digit multiplications in cases - math is relatively easier there.
  • If you do encounter a 3-digit multiplication then try to simplify it by cutting corners. 351 * 258 can be approximated as 350 * 260. Which can be seen as 700 * 130. Which can further be written as 7000 * 13. Then as 7000 * 10 + 7000 * 3. Then as 70000 + 21000 = 91000. The actual answer for 351 * 258 is 90,558. So not far off.
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