> (e.g. `9 x N` is initially taught as `10 x N - N`) That isn't a crutch, that is how large numbers can be multiplied in one's head! 90 * 21 is (90 * 20) + 90

> (e.g. `9 x N` is initially taught as `10 x N - N`) That isn't a crutch, that is how large numbers can be multiplied in one's head! 90 * 21 is (90 * 20) + 90 Right after removing that extra 90, now you have 9 * 2 * 10 * 10 + 90, which is easy to do mentally. Heck if you ask me what 9 * 14 is, I'm going to do 10 * 14 - 14. Did I have to memorize 0x0 through 12x12? Yup. But now days I have a few key mid points memorized and I'll add my way from there. I don't remember 7 * 6 but I know its 7 * 7 - 7.