I just found this https://en.wikipedia.org/wiki/Trachtenberg_system

*“The Trachtenberg system is a system of rapid mental calculation. The system consists of a number of readily memorized operations that allow one to perform arithmetic computations very quickly. It was developed by the Russian Jewish engineer Jakow Trachtenberg in order to keep his mind occupied while being held in a Nazi concentration camp.”*

When I was eight I came up with a really quick way of instantly multiplying any number by 99 in my head which is really similar. Didn’t write out the ‘formula’ at the time obvs but it was just

a x 99 = FIRST DIGITS: [a -1] SECOND DIGITS: [100-a]

e.g. 66 x 99 = 6534

89 x 99 = 8811

remember telling my teacher and she was like ‘oh you’re just multiplying by 100 and taking one very good’ and didn’t get what I was trying to explain at all.