Ask HN: Best shortcuts for doing math in your head

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