2 ms·

I can't speak for anyone else, but for my mental math (which admittedly is only so-so, not spectacular), I make pretty liberal use of factoring. Want to multip

• ColinWright 5y ago
  Or if you want to multiply by 15, multiply by ten, then add on half again. So 34 times 15 is 340 plus half of 340, which is 170, so the answer is around 500 (actually 510). But if you want 34 (specifically) times 15 there are other ways to go. Or (34 times 15) is (17 times 30). Six times 16 is around 100, so three times 15 will be half that, or around 50. So 34 times 15 is around 500. Or 34 is about 100/3, so multiply that by 15 is 100/3 times 15, which is 100 times 15/3, which is about 500. Being good at mental arithmetic and estimation isn't about being able to do sums quickly and accurately, it's about being happy with rough calculations, retaining a sense of how accurate you are so you can fix it later (if necessary).
  • jonsen 5y ago
    > So 34 times 15 is ... We who knows our powers of two do 34 ~= 2^5 15 ~= 2^4 2^5 * 2^4 = 2^9 = 512
    • tombert 5y ago
      I'm embarrassed to admit that I know my powers of two pretty well, and I didn't even think of this trick.