In my head, 19+25+28 becomes: 20 + 25 + 30 - 3 = 72 Works well with multiplication too. 12 * 13 = 10 * 13 + 2 * 13 = 130 + 26 = 156. or 19 * 15 = 20 * 15 -

In my head, 19+25+28 becomes: 20 + 25 + 30 - 3 = 72 Works well with multiplication too. 12 * 13 = 10 * 13 + 2 * 13 = 130 + 26 = 156. or 19 * 15 = 20 * 15 - 15 = (10 * 15) * 2 - 15 = 285 There's no simple rule I know for multiplying by 19, but for 20 you just add a zero and double it. It's all about transforming the numbers into ones that are easier to work with in your head, and just accounting for your transformations at the end. Even if you forget to account for your padding, you're still in the right ballpark and have a rough idea of what the answer should be near.