[gordaco]

That's funny. I have a degree in maths, which I guess qualifies me as a mathematician, although I've always worked as a software developer.

So, the thing is that I'm not a good mathematician, but I happen to be good at arithmetic. By this I mean that I'm good with mental math (of the "square a 4-digit number in ten seconds or less" type; I'm not nearly as good as the savants out there who perform operations with many more digits, and much more quickly), but also that I like playing with numbers and I've came up with a ton of silly "theorems" [1] (I'm not sure if they even deserve that name) that are mostly based on basic modular arithmetics, so any actual mathematician might find them amusing but nothing more, while a layperson is often amazed. So, mental arithmetic can be useful to make people believe that you are smarter than you actually are :) . I'm not fond of doing this, by the way. But it's a thing that happens.

[1] Here goes an example. Take a number that is a multiple of 73, that has exactly seven digits, and that has a zero somewhere in the middle. Say, 73*75391 = 5503543. Then you can exchange whatever goes before and after the zero, and the result is also a multiple of 73: 3543055/73 = 48535. For added WTF, I'd like to mention that it also works with 137: 137*8621 = 1181077, and then, 7701181/137=56213. The proof is surprisingly simple, but you need to know what to look for.

[openknot]

A great book for this is "Secrets of Mental Math" by Benjamin and Shermer [0]. I remember a feeling of fun when I memorized the tricks many years ago, but unfortunately I lost the digital flash card deck for reviewing them, so I've forgotten them now. I should recreate it.

[0] Goodreads link: https://www.goodreads.com/book/show/83585.Secrets_of_Mental_...

[ziml77]

I came up with my own shortcuts for mental math back when I was in middle school. But since then whenever I've needed to do anything more than a simple calculation I've just reached for a calculator and have basically lost the ability to do the longer work in my head. While it would have been nice to retain that ability without continued practice, pulling out my phone's calculator is simply faster and won't make an error like I can easily in my head when trying to remember the all the intermediate numbers I'm calculating.

[openknot]

Calculators are preferred for important calculations, though upon further thought, I remember reading that mental math is prized in the finance sector.

Out of curiosity, a search on r/FinancialCareers provided anecdotes as some evidence that it mostly helps you seem smart to higher-ups, and also does have a niche role in prop trading [0]:

1. "Prop trading firms highly value quick mental math. I work at a prop in Chicago, specifically market making in the broker market on CME products, and very fast mental math is required as part of my daily job. I calculate my hedges using mental math, and it’s absolutely crucial I’m quick (futures can move fast). The mental math I do daily is mostly multiplication and division of two-digit numbers, as well as some fraction multiplication. Although prop trading firms are few and far between now, so it’s a niche field and a skill with little required use outside of trading."

2. "Youll find that aa lot of politcal wins are made in meetings with higher-ups. These people often have little time and want things boiled down to a mental math state (they dont have time for you to fix your model in real time). This is where quick mental math can get you a good impression. If an exec has a question surrounding the effect of something that isnt in your model, you will need to do quick mental math to figure it out. This exec doesnt have time for you to do more than 5 key strokes. It is important as it makes you seem like a thought leader."

3. "Extremely relevant. Will help you impress in internal and external meetongs"

[0] https://old.reddit.com/r/FinancialCareers/comments/c7c38l/is...

[donkeybeer]

Quick estimation and sanity checking are useful skills even if the real program is going to be much more complex.