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Mental Maths: Best way to do division?

This is especially for certain chart interpretations where we have to find the percentage growth across periods

So for example, if something is growing at a rate of 60/130, what I do in my head is say 60/120 is 50%, so if the denominator is increasing by a bit, then the number should be lower to around 40%

But I know this way isn't exactly "correct" so im struggling to figure out the best method for this that won't take forever

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Top answer
Joel
Coach
on Jul 20, 2025
Buy 1 get 1 free (July only) | Kearney | Ex-RB | Involved in recruiting | Passed 10/10 interviews | 250+ interviews

Hello,

I really like your thinking; you’re already heading in the right direction.

Here are a few tips to keep in mind as you continue practicing:

  1. Simplify the calculations: When performing a complex calculation, make it easier by asking to round up or down, but do so properly. The best practice is to clearly state what you're rounding to and justify it by noting that the difference is no more than ~5%, so it won't significantly affect the analysis. This shows you're being pragmatic, not careless.
  2. Present your methodology first: Before diving into detailed math, explain your approach and confirm it with the interviewer so you can get them on board.
  3. Talk through your math out loud: Verbalizing your calculations allows the interviewer to follow your logic, offer guidance if needed, and gives you a chance to impress them.

For this specific case, and to build on Kevin’s solid approach, Step #2 can be simplified as follows:
5 ÷ 130 = 0.5 ÷ 13 = 1 ÷ 26 ≈ 1 ÷ 25 = 4%

Hope this helps! Feel free to reach out if you have any follow-up questions.

Best, 

Joel

Kevin
Coach
on Jul 20, 2025
1st session -50% | Ex-McKinsey | Ex-BCG | MBB Germany | PEI Expert | CV & Cover Letter Review | FREE 15min intro call!

Hi there,

Your idea to find an easy approximation is already very good! Let me share how you can do this a bit more precisely: Instead of adjusting the denominator, just change the numerator to make the math easier. Here’s how I’d do it:

Let’s say you have 60 ÷ 130. That looks annoying at first, but you can follow 3 simple steps:

  1. Add 5 to the 60. Why? Because 65 ÷ 130 is exactly 50%.
  2. Now, adjust for that extra 5 you added. What’s 5 ÷ 130? That’s just under 4%.
    Here’s a trick to estimate that quickly:

    Think of 5 ÷ 130 as 10 ÷ 130, divided by 2.
    10 ÷ 130 is like 1 ÷ 13, which is around 7.7%.
    Half of that gives you roughly 3.85%.

  3. Now subtract that from the 50% you had at the start.
    So, 50% – just under 4% = just over 46%, which is pretty much the correct answer (actual value ≈ 46.15%).

Bottom line: Do an approximation to an easier number, but adjust the numerator instead of the denominator. 

Bonus tip for interviews: Memorise common fractions - things like 1/4 = 25%, 1/5 = 20%, 1/6 ≈ 16.7%, 1/11 ≈ 9%, 1/13 ≈ 7.7%. That’ll save you tons of time in interviews and make you look like a mental math pro without much effort!

Feel free to book a free intro call with me to learn how you can effortlessly navigate through math problems during cases!

Hope that helps.

Cheers,

Kevin

Hagen
Coach
edited on Aug 04, 2025
#1 recommended coach | >95% success rate | 9+ years consulting, interviewing and coaching experience

Hi there,

I would be happy to share my thoughts on your question:

  • First of all, your current method is already heading in the right direction, but I would advise you to shift your mental shortcut slightly: instead of adjusting the denominator, try adjusting the numerator. For example, in your 60/130 case, think "65/130 = 50%", then subtract the 5 you added. Since 5/130 ≈ 3.8%, that gives you an estimate of ~46.2%.
  • Moreover, I would advise you to memorize key fractions you’re likely to encounter in case interviews. That mental library will help you approximate ratios in seconds.
  • Lastly, please keep in mind that, except for very simple calculations, you are not expected to perform them mentally; rather, you may do them in written form. Therefore, I would strongly advise you not to worry too much about mental math, as you will naturally become proficient in it through practicing written calculations anyway.

You can find more on this topic here: Speed reading techniques.

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

Best,

Hagen

on Jul 21, 2025
#1 Rated McKinsey Coach | Top MBB Coach | Verifiable success rates

First of all, most candidates overestimate the importance of mental maths. For consulting firms it matters a lot less than most prep materials lead you to believe.

Second of all, doing long division on paper is a perfect way of solving calculations. You don't always need to give a rounded number. You can actually calculate the accurate number. 

The approximation you suggested works fine, but once again, it's not what's actually important to signal in case interviews.

If you're practicing case math, you might also find this resource helpful:


Best,
Cristian

Mariana
Coach
on Jul 22, 2025
xMckinsey | Consulting and Tech | 1.5h session | +200 sessions | Free 20-min introductory call

Hi there,

Besides what has been said: Memorize fractions until 1/15. That will help a lot.

Also, you can multiply everything by a common factor if it makes the calculation easier. 

Usually there is a easy way to solve case problems, and interviews will be pleased to see you discovering it, so good job on trying to find ways to achieve that!

Best,

Mari

Alessa
Coach
on Jul 23, 2025
xMcKinsey & Company | xBCG | xRB | >400 coachings | feel free to schedule an intro call for free

Hey!!

Your approach is actually great, ballpark estimation is exactly what interviewers want: fast, structured, and directionally accurate. You don’t need exact precision unless they ask for it.

For cleaner division, try this mental trick:
60/130 = (6/13) → double both → 12/26 ≈ 0.46 or 46%

Also helpful: memorize common splits like
1/3 = 33%, 2/3 = 66%, 1/5 = 20%, 3/8 = 37.5%, etc.
These shortcuts help in % change or share questions.

best,
Alessa :)

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