Cookie and Privacy Settings

This website uses cookies to enable essential functions like the user login and sessions. We also use cookies and third-party tools to improve your surfing experience on preplounge.com. You can choose to activate only essential cookies or all cookies. You can always change your preference in the cookie and privacy settings. This link can also be found in the footer of the site. If you need more information, please visit our privacy policy.

Data processing in the USA: By clicking on "I accept", you also consent, in accordance with article 49 paragraph 1 sentence 1 lit. GDPR, to your data being processed in the USA (by Google LLC, Facebook Inc., LinkedIn Inc., Stripe, Paypal).

Manage settings individually I accept
expert
Expert with best answer

Francesco

100% Recommendation Rate

3,265 Meetings

11,659 Q&A Upvotes

USD 429 / Coaching

7

How fast (sums/minute) should you be able to do mental math?

Practicing on the Preplounge maths tool, how many 'hard' multiplications should you be able to do in your head without using paper? In interviews, will firms just give you a cold sum and ask you to do it in your head - how long is too long and how long is quick for say, 27*86 ?

Practicing on the Preplounge maths tool, how many 'hard' multiplications should you be able to do in your head without using paper? In interviews, will firms just give you a cold sum and ask you to do it in your head - how long is too long and how long is quick for say, 27*86 ?

7 answers

  • Upvotes
  • Date ascending
  • Date descending
Best Answer
Book a coaching with Francesco

100% Recommendation Rate

3,265 Meetings

11,659 Q&A Upvotes

USD 429 / Coaching

Hi there,

I agree with Henning, you don’t need to perform math in your head unless the interviewer explicitly asks to do so, which is rare.

If you really have to do math in your head due to the interviewer's request, my suggestion is to increase the number of steps to simplify the math. You will be slower but far more accurate, which is always better:

27*86 --> 30*86 – 3*86 = 30*86*0,9 = (30*90 – 30*4)*0,9 =(2700 – 120)*0,9= 2580 – 258 = 2322

Best,
Francesco

Hi there,

I agree with Henning, you don’t need to perform math in your head unless the interviewer explicitly asks to do so, which is rare.

If you really have to do math in your head due to the interviewer's request, my suggestion is to increase the number of steps to simplify the math. You will be slower but far more accurate, which is always better:

27*86 --> 30*86 – 3*86 = 30*86*0,9 = (30*90 – 30*4)*0,9 =(2700 – 120)*0,9= 2580 – 258 = 2322

Best,
Francesco

Book a coaching with Robert

96% Recommendation Rate

317 Meetings

3,639 Q&A Upvotes

USD 219 / Coaching

Hi Anonymous,

In the first place, make sure that you end up with the correct result.

5 seconds and wrong result = fail

15 seconds and correct result = pass

Obviously you need to show that you are comfortable with numbers and that you can do math. If you can do it mentally, great. If not, use paper and give me the result. Since this is something simple to train you won't get a lot of bonus points any way, just don't do it wrong.

Hope that helps - if so, please be so kind and give it a thumbs-up with the green upvote below!

Robert

Hi Anonymous,

In the first place, make sure that you end up with the correct result.

5 seconds and wrong result = fail

15 seconds and correct result = pass

Obviously you need to show that you are comfortable with numbers and that you can do math. If you can do it mentally, great. If not, use paper and give me the result. Since this is something simple to train you won't get a lot of bonus points any way, just don't do it wrong.

Hope that helps - if so, please be so kind and give it a thumbs-up with the green upvote below!

Robert

Book a coaching with Henning

100% Recommendation Rate

138 Meetings

3,893 Q&A Upvotes

USD 169 / Coaching

I would not do 27*86 in my head. It's perfectly fine to do this in writing. In fact, better do it on paper and get it right, than do mental math and get it wrong.

Of the 120 actual interviews I've done, there was probably only 1 or 2 candidates that impressed me with mental math. And even for them, it did not move the needle of the overall case assessment. On the flipside there were numerous candidates that were too ambitous and got thrown off course by messing up mental math.

My recommendation: don't aim for speed, but for accuracy. Know your capabilities and play defense, not offense when it comes to the quant part.

I would not do 27*86 in my head. It's perfectly fine to do this in writing. In fact, better do it on paper and get it right, than do mental math and get it wrong.

Of the 120 actual interviews I've done, there was probably only 1 or 2 candidates that impressed me with mental math. And even for them, it did not move the needle of the overall case assessment. On the flipside there were numerous candidates that were too ambitous and got thrown off course by messing up mental math.

My recommendation: don't aim for speed, but for accuracy. Know your capabilities and play defense, not offense when it comes to the quant part.

Book a coaching with Oscar

100% Recommendation Rate

15 Meetings

93 Q&A Upvotes

USD 149 / Coaching

Hello, The question is very simple to be honest as all my referalls are passing a rigourious time math calculation pressure and training. Despite comments the best is to calculate it in your head still the problem is that the level of stress that you will find during your interview will be very high and will cause issues to concentrate. This is why you need to practice a lot just to have a confort that the calculation methodology will be automatic for you. The time of calculation is as follow :


Beginner - 25 seconds or more

First week of pracitce - around 15 seconds

2-3 weeks of practice - around 12 seconds

4+ weeks of practice - around 10 seconds

I always recommend to enter the interview with a timing below 10s. My average score when I was entering the intererview were 6-7 seconds (None of my referalls have reached this level yet).

I hope it's practical. Let me know if you would like to meet and understand my methodology of calculation and the training approach.

Sincerely,

Oscar

Hello, The question is very simple to be honest as all my referalls are passing a rigourious time math calculation pressure and training. Despite comments the best is to calculate it in your head still the problem is that the level of stress that you will find during your interview will be very high and will cause issues to concentrate. This is why you need to practice a lot just to have a confort that the calculation methodology will be automatic for you. The time of calculation is as follow :


Beginner - 25 seconds or more

First week of pracitce - around 15 seconds

2-3 weeks of practice - around 12 seconds

4+ weeks of practice - around 10 seconds

I always recommend to enter the interview with a timing below 10s. My average score when I was entering the intererview were 6-7 seconds (None of my referalls have reached this level yet).

I hope it's practical. Let me know if you would like to meet and understand my methodology of calculation and the training approach.

Sincerely,

Oscar

Book a coaching with Adi

100% Recommendation Rate

47 Meetings

3,960 Q&A Upvotes

USD 149 / Coaching

Sometimes its also perfectly okay to round up/down slightly to make the math easier. Make sure you validate this with the interviewer if you can, otherwise just do it and make the interviewer aware. You wont be penalised in the grand scheme of things if your answers are 5-10% within error. But use this style with caution depending on the case and situation.

e.g 27*86 could be 25*85

Personally when am interviewing I do give some slack to candidates to simplify the math but it will vary from interviewer to interviewer and company to company.

Sometimes its also perfectly okay to round up/down slightly to make the math easier. Make sure you validate this with the interviewer if you can, otherwise just do it and make the interviewer aware. You wont be penalised in the grand scheme of things if your answers are 5-10% within error. But use this style with caution depending on the case and situation.

e.g 27*86 could be 25*85

Personally when am interviewing I do give some slack to candidates to simplify the math but it will vary from interviewer to interviewer and company to company.

Book a coaching with Allen

100% Recommendation Rate

44 Meetings

767 Q&A Upvotes

USD 259 / Coaching

The bottom line is that if you are uncomfortable with your speed, just use pen and paper.

Consultancies are testing for comfort with numbers, not mental math. So you really lose nothing if you do it on paper. On the other hand, if you make a mistake or choose to simplify in order to make the calculations easier, you can sidetrack your case.

As usual, think about real life. Is there ever a situation where you need to be able to preform difficult mental math quickly and can't access a cellphone or a pencil? Not really. But there are many in which you have to do it accurately. That's what is being tested.

The bottom line is that if you are uncomfortable with your speed, just use pen and paper.

Consultancies are testing for comfort with numbers, not mental math. So you really lose nothing if you do it on paper. On the other hand, if you make a mistake or choose to simplify in order to make the calculations easier, you can sidetrack your case.

As usual, think about real life. Is there ever a situation where you need to be able to preform difficult mental math quickly and can't access a cellphone or a pencil? Not really. But there are many in which you have to do it accurately. That's what is being tested.

Book a coaching with Ian

100% Recommendation Rate

185 Meetings

15,611 Q&A Upvotes

USD 289 / Coaching

First: Get the math done right

Second: You don't have to do all fast math "in your head". Write this one down! Split it up into segments if possible

Third: Learn long division + multiplication...this one honestly might be better done as long division

Finally:

I found the following useful

  • Online "Drills": https://www.jetpunk.com/quizzes/fast-math-multiplication-quiz
  • Math sheets (print these and do them on paper): https://www.math-drills.com/

First: Get the math done right

Second: You don't have to do all fast math "in your head". Write this one down! Split it up into segments if possible

Third: Learn long division + multiplication...this one honestly might be better done as long division

Finally:

I found the following useful

  • Online "Drills": https://www.jetpunk.com/quizzes/fast-math-multiplication-quiz
  • Math sheets (print these and do them on paper): https://www.math-drills.com/