Pen and paper - how often to use for mental calcs?

Math problem
New answer on Oct 08, 2022
9 Answers
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Nick asked on Oct 05, 2022

I heard that in some consulting companies they would prefer that on the interview you frequently use pen and paper and show all the steps of your thought and calculations. But in other companies they wouldn't like it for short questions, not complicated Math calcs. They would prefer giving them the answer orally almost right away.

Which specific companies (Big-3) prefer which approach?

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Dennis
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replied on Oct 05, 2022
Seasoned project leader with 7+ years of consulting and recruiting experience in USA and Europe

Using pen and paper should help the process of working towards the solution you are presenting in a clear and concise manner. If it becomes an extra step that just slows you down in giving the answer to a short question, it is obviously not very well utilized. Overall, you want to make sure that the interviewer understands where your head is, that you can easily backtrack if needed and that you yourself don't get lost in a potential numbers jungle.

During Covid, there was a time when all interviews were virtual and the interviewer could not even see the candidate's notes. So I don't believe that any firm is strictly set on anyone approach at this point. Be pragmatic and tailor it to the situation. But when in doubt, I would rather write something down for documentation than “juggling” around with numbers and structures

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Emily
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replied on Oct 08, 2022
Ex McKinsey EM & interviewer (5 yrs) USA & UK| Coached / interviewed 200 +|Free 15 min intro| Stanford MBA|Non-trad

It is absolutely fine to use a paper and pen to do calculations - consulting firms are looking for people who are comfortable with numbers, not for human calculators. That being said, if you're able to do calculations without it, then that's fine too. 

I would be very surprised though if you could get through the whole of the maths part of the case without having to write down, at the very least, the methodology that you're following to solve the question. Write down what you need to write down to keep you on track and stop you from making silly errors.

Good luck! 

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Sofia
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replied on Oct 07, 2022
McKinsey San Francisco | Harvard graduate | 5+ years of coaching| Free 15 min intro call | Personalized approach

Hello,

I agree with the general feedback you have received here - there is no one right or wrong answer. If a calculation is straightforward and you can do it in your head, go for it. If it requires several steps and would be cleaner and easier to follow if you write it out on paper (+ would decrease the chance of you making a mistake), then write it out. Generally most quantitative question that you get in cases will fall under the latter bucket.

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Pedro
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replied on Oct 07, 2022
Bain | EY-Parthenon | Roland Berger | FIT | Market Sizing | Former Head Recruiter

Correct answer: they don't care.

They care that you are able to adequately explain your process, and able to perform tasks in an efficient way. 

If they are not able to follow you there's a problem with communication. If you are being too mechanic and “bureocratic” when approaching any problem, then you are not being efficient.

So you have to apply judgement. Good judgement is around the lines of: if you need to formula with more than 3 steps you should write it down. Otherwise you should be able to communicate and calculate effectively without resorting to paper.

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Ian
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replied on Oct 07, 2022
MBB | 100% personal interview success rate (8/8) and 95% candidate success rate | Personalized interview prep

Hi Nick,

It's all contextual. It depends on the case (and, honestly, interviewer).

Let's keep this rule very simple:

  1. If you can not take time and solve the problem in a correct, structured, clear way, then don't take time.
  2. If you need to take time, because otherwise you will have an incorrect, messy, unclear answer, then take the time you need.

Your question is a false choice :)

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Mario
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replied on Oct 06, 2022
Ex-Mckinsey (analyst->associate->manager) and now in tech (Bytedance) + Part time interview coach and mentor

Hi there,

 

For quant questions, what matters is you structuring the math right, taking shortcuts and landing on the right numbers. HOW you do that can vary and isn't the core of the assessment. 

 

Practically speaking, when given a calculation to do, here is how you should deal with it:

1- If the calculation is straightforward and can be done mentally, and if you feel very confident about the answer, you should go ahead and give the answer right away, without justification.

2- If the math requires a bit more structuring and you do not have the answer at the top of your head, do structure it on paper (e.g., decompose, use shortcuts etc.,). If it's a simple structure, no need to walk the interviewer through it, just go ahead and give the answer. If the structure is complicated and takes time to solve, you can walk the interviewer through it to ensure it's correct and avoid awkward silence.

 

All in all, whenever possible answer the question directly, whenever complicated do write things down.

 

Best of luck,

 

Mario

 

 

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Florian
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replied on Oct 06, 2022
#1 rated McKinsey Case and PEI Coach | 5 years at McKinsey | Mentorship Approach | 120+ McK offers in 18 months

Hi there,

there is no preference here.

I would always use pen and paper for the sole reason that you want to have a paper trail of your thoughts and calculations.

If you end up with a wrong result, it gets tedious to go back to your calculations as there is no record of it.

Additionally, you appear more structured AND organized, which also helps with the outcome.

Cheers,

Florian

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Dominico Frantelli replied on Oct 05, 2022

I heard smth like this in Europe. First case is favoured in McKinsey, the second in BCG. Don't know about Bain though

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Maikol
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replied on Oct 05, 2022
Incoming BCG Principal | Former Bain, AlixPartner, and Private Equity | INSEAD MBA | GMAT 780

I think you can ask many questions to improve your performance, and this is not among them.

You have to use your pen and paper only when is of help to you. 

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Dennis gave the best answer

Dennis

Seasoned project leader with 7+ years of consulting and recruiting experience in USA and Europe
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