This would have made my college career significantly easier. We were limited on scratch paper (dumb policy) on many tests, and I have to write out arithmetic like this long-hand style, which took up a ton of space.
The one everyone knows is calculating a tip (bump the decimal place left once and double the result, round depending on how nice you feel). Might be applied multiple times per day, depending on your dining habits.
I bet the formulae you had to use on your tests were worth doing by hand.
(US-defaultism?)
(EU guy who has worked as a waiter)
Luckily I got to use a calculator for school.
[0]https://archive.org/details/dli.ernet.11725/page/n9/mode/1up
When I first encountered the outrage over New Math my first thought was that this is how I avoid embarrassing myself in checkout lines. Do I have enough cash to pay for this stuff in my hands?