I'm pretty good at math in my head, I thank being a cashier for that.

While I can't use a slide rule (but I can do long division), I did use it once...

Had a chemistry final in college. Professor said no calculators. I promptly borrowed my dad's slide rule, sat in the front rule, and fiddled around like mad trying to [censored] off the teacher ( I wasn't fond of her "office hours" as her answers tended to be "read the book." B!%ch. I'm still pissed, 15 years later)