
Ask HN: Best shortcuts for doing math in your head - cVwEq
What are some of your &quot;go-to&quot; mental shortcuts for doing math in your head, that you use regularly?<p>A few examples that have been useful to me:<p>1. Memorizing fractions to decimals for 1&#x2F;1 through 1&#x2F;25. (e.g. 1&#x2F;7 = approx 0.14. Then if you have, say, 3 out of 7 of something, it&#x27;s easy to approximate the percentage or do other useful things.)
2. A number is evenly divisible by three if the sum of its numbers is evenly divisible by three (e.g. 3213; 3 + 2 + 1 + 3 = 9, which is evenly divisible by three, therefore, 3213 is as well)
3. Memorizing 2^x from x=0 to 22
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kirankn
Try to get hold of this Indian book: Vedic Mathematics or Sixteen Simple
Mathematical Formulae from the Vedas
([https://www.amazon.com/gp/product/8120801636/](https://www.amazon.com/gp/product/8120801636/)).

I have also heard of a Trachtenberg Speed system
([https://www.amazon.com/Trachtenberg-Speed-System-Basic-
Mathe...](https://www.amazon.com/Trachtenberg-Speed-System-Basic-
Mathematics/dp/4871877094)).

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celticninja
For children? Nine times table on your fingers is good.

[https://youtu.be/xBTGKiVgWcA](https://youtu.be/xBTGKiVgWcA)

