I had my first checking account when I was about thirteen, a credit card in my early twenties, and a debit card not long after that. I've never carried around a lot of cash. I remember seeing people who would get paid and take a roll of cash with a rubber band around it, and spend it that way. That would just confuse me. But this year I've decided to try to spend cash, in bills and coins.
Luckily, this is the good old USA, which has a very easy system, based on decimals. And even I can do decimals! The basic unit is a dollar, which is divided into one hundred pennies. And if bills and coins had been created in units of ten, it would have been pretty darned easy. Unfortunately, they aren't. There is, of course the dime, which is ten pennies, or 1/10th of a dollar. And then there are quarters, which divides a dollar into fourths, and nickels, which divided a dollar into 20ths. When I was a kid, there were fifty-cent pieces, but I don't see them anymore. So, in a perfect decimal world there would be pennies (100ths), dimes (10ths), fifty-cent pieces (1/2), and dollars. Then there would be tens, then hundreds. Of course, those are too big of a jump so the middle bills and coins are included. Oddly enough, instead of a $25-dollar bill, to match a quarter, US. bills are in twenties. No one asked me, but it seems like they could have made coins and bills consistent, but they didn't.
The ability to quickly look at these things and add them up is something that I never worked on. I was also one of those kids who couldn't calculate batting averages of baseball players, or do point spreads. Yeah, I've stayed away from Las Vegas! I did learn to play Black Jack, which helped me a bit with practicing adding quickly in my head, but I didn't stay with it, and nowadays I'm embarrassed to play it, and when my hand is "busted" it takes me quite a while to add it all up. I marvel at people who can do it almost instantaneously, as they slap their cards down and say "busted!"
As you tell can tell, it's the arithmetic of all of this that gives me a headache. I can do math, including algebra and geometry, but I have to write stuff down, I can't do it in my head. And that's the challenge of dealing with money in bills and coins, you have to do the math.