Earlier quoted context omitted.
>But he doesn't elaborate on it further, and neither does anyone else from what I can tell, and I have yet to figure out the trick 5 * 9 = 5 * (10 - 1) = 50 - 5 = 45 Specifically, you need to know everything in the 5x5 area, and how to multiply by 10. More generally, any digit 6-9 can be rewritten as (10 - (10 - digit)) and then you can apply distribution rules to clean things up. Here's a three by two digit example:…
I think this is one instance where using a larger look-up table (i.e. multiplication table up to 10x10) yields clear benefit. The technqiue using only 5x5 increases the number of intermediary results to keep in one's head.
The grandparent poster's example only looks more complicated because we don't have standard glyphs for the negative digits.