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Mental Math vs. Paper Math? At what level of difficulty is it OK to switch from mental math to paper?

Anonymous A asked on Nov 30, 2018 - 4 answers

I've read various different opinions on the amount of mental maths required for case studies. For example, I know double-digit multiplications need to be done efficiently, but would it be expected to calculate all double-digit multiplications using only mental maths? Or would it be okay to write out higher-number combinations on paper? (such as 78*83)

Any comments on this topic would be much appreciated.

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replied on Nov 30, 2018
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I don't think there's any hard & fast rule. If I feel like the math is getting a little more complex, or I have too many numbers to keep track of, I always err on the side of caution: better go a bit slower & be accurate, than rush mental math calculations and make an expensive mistake. Where the threshold is depends on your own skills... and stress levels

replied on Dec 03, 2018
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Hi Anonymous,

there is a lot of damaging advice floating around on consulting prep forums, all focused on "increase your speed of mental math".

The truth is, you don't get bonus points for being lightning fast - quite at the contrary! The interviewer will assume that the way you behave in an interview is also how you behave vis a vis a client. And the single most important thing when discussing with a client is that you can be followed easily! If you are super well-trained in mental math and do all sorts of complicated math stunts in your head - guess what the result will be? Your counterpart will have difficulties following your steps. Hence, you are not easy to follow - and this is YOUR fault, not your counterparty's!

So what is far more important than speed is the rigour and discipline to walk the interviewer through your calculation in a succinct, sharp and cystal clear way! I will always recommend to do calculations in written form and neatly copy the results into a seperate area on the sheet (or another sheet). This is 100x more effective than trying to become super fast in mental math!

Cheers, Sidi

Anonymous B updated his answer on Nov 30, 2018

Some offices expect you to be more proficient in math than others, but even those that have higher expectations won't penalize you for somewhat slow math. There was a time when a manager during McKinsey interview asked me why it took me so long to calculate, I just said I didn't want to make a negligent mistake by missing a zero digit and arriving at 10x wrong result. I passed that interview.

You are not expected to perform a double-digit multiplication in you head.

It's expected that you'll be efficient by rounding numbers, finding shortcuts and not spending too much time trying to arrive at a precise result. For 78*83 they'll expect that you'd suggest to use 80*80=6,400 instead but can do a more precise calculation if need be. There is a 1% rounding error here, which is totally negligible and will allow you to arrive at the same "so what" conclusion as the 78*83=6,474.


replied on Dec 02, 2018
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You can do it on paper, but i recommend using this method for faster calculations:


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