I think these sort of mental math tricks are mostly useful when you have a case where you’ve got a formula that you often have to run, and it is similar enough

I think these sort of mental math tricks are mostly useful when you have a case where you’ve got a formula that you often have to run, and it is similar enough every time that you can pre-do the algebra. 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.