I went to college in 1973 and I started learning the slide rule for fun in high school. I never did become a whiz at it like my dad. (There’s a trick for estimating the 3rd digit when doing large-number multipilcation) Calculators were available by then - I bought one with 4-functions for $100 in spring 1974 and a full scientific for $29 only 3 years later.
The key to a lot of the advanced math is memeorizing the math tables. Forced rote memorization seems to be something modern teaching methods have trouble with, but it’s something you cannot avoid to be proficient.
What’s really needed is a class on estimating: “Joe has $137,980 and offers you 16% - how much roughly is that?” Your brain should say “That’s about $14,000 for 10%, 5% more is another $7,000 for $21,000. Plus 1% is $1,400 and I rounded up; so about $22,000.”
If your geekitude comes into play, you can estimate 16% of $140,000-137,980 =$2,020 and subtract that from $22,400.(16x2=32, so about $320 plus $3.20; etc.) And amaze your friends with your mental math before they finish getting their cellphone out, switching to calculator, and keying it in.
So many people use a calculator, make a typo (or multiply instead of dividing) and come up with a number that’s wildly off base, and don’t realize that an answer around $2,000 or $180,000 could not possibly be right.