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Fast mental math is useful enough that folks in different fields keep reinventing variants of it, with different terminology:

- Estimation ('market sizing') is a standard part of management consulting company case interviews [1]. The reason is because clients will be throwing you questions and one part of the job is to look smart and give reasonable answers on the fly, without going to a computer or grabbing a calculator first.

- Physicists call them Fermi problems [2]

- Microsoft (in)famously asked 'How many ping pong balls fit into a 747?' as a brain teaser [3]. This was common enough that someone wrote a book about these brain teasers [4].

- Fast mental math is a standard part of many trader interviews, since you'll be making split-second decisions under pressure [5]

One technique is converting everything into log10 first, e.g. 3 billion is about 3 * 10^9 ~ 10^9.5, then you're just adding / subtracting exponents to multiply / divide. Another way is to always round inputs to 'easy' numbers (2, 3, 5), and calculate them separately from exponents.

A few minutes with a napkin can easily save several hours doing something that can't possibly be worthwhile [6]

[1] https://mconsultingprep.com/market-sizing-example/

[2] https://en.wikipedia.org/wiki/Fermi_problem

[3] https://www.inc.com/minda-zetlin/microsoft-changes-job-inter...

[4] https://www.amazon.com/How-Would-Move-Mount-Fuji/dp/03167784...

[5] https://www.quora.com/Why-do-hedge-prop-quant-funds-ask-ment...

[6] https://xkcd.com/1205/

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