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Ask HN: Best shortcuts for doing math in your head
What are some of your "go-to" mental shortcuts for doing math in your head, that you use regularly?
A few examples that have been useful to me:
1. Memorizing fractions to decimals for 1/1 through 1/25. (e.g. 1/7 = approx 0.14. Then if you have, say, 3 out of 7 of something, it'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
- celticninja 8y agoFor children? Nine times table on your fingers is good. https://youtu.be/xBTGKiVgWcA https://youtu.be/xBTGKiVgWcA
- kirankn 8y agoTry 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-Mathematics/dp/4871877094 https://www.amazon.com/Trachtenberg-Speed-System-Basic-Mathe...).