Tangentially related, Jakow Trachtenberg invented a new and much faster system for mental arithmetic [1] while imprisoned in a Nazi concentration camp. It's a far cry from Common Core math, not easy to learn but once you know it, wow, you can multiply arbitrary 3-digit numbers in your head nearly instantly. 4-digits and higher gets a little trickier but the mental difficulty increases slowly.
[1] https://en.m.wikipedia.org/wiki/Trachtenberg_system