Some people shouldn't be handling money
From HappyFunPundit:

Tales of Mathematical Inadequacy
...The total was $14.72. I handed the young woman a $20, and waited for my change.
Then disaster struck. As the woman pressed the 'total' button, a look of fear and confusion swept across her face. A look not unlike the one the cat used to get when I came home with a bag of hot sauce. This girl was clearly troubled. Nay, frightened.
"What's the problem?" I asked.
"The cash register! It's.. It's not giving me the amount of money you need. It must be broken." she replied changelessly.
A quick subtraction in my my head. "You owe me $5.28".
The girl looked up, startled. A number? The man just said a number? How could he know? After all, the machine won't give the number! "Uh, I better find a calculator."

The story gets better -- although, as Professor Reynolds points out, it would be funnier if it weren't true. I remember when one of my college roommates wondered out loud what ten times eleven was. Without thinking, I said "one hundred and ten."

"What?"

"One hundred and ten. Ten times eleven. It's one hundred and ten."

"Let me just get a calculator and check that..."

You do that, partner. You just do that. And think about how much more time you'd have on your hands if you didn't have to struggle with ten times eleven.

Math is not that hard, yet businesses insist on putting people in charge of their cash registers who haven't mastered it. Is there a shortage? Look, making change is the easiest thing in the world to do once you learn the trick. You don't subtract, you add.

"$14.72 out of twenty, that's..." It ends with a number other than zero and five, so you know you'll need pennies. Grab some and start counting up from what the customer owes you as you drop them into your other hand. "...seventy-three (plink), seventy-four (plink), seventy-five (plink)..." I don't think it's asking too much to memorize those numbers that are only one coin from a round dollar. Grab a quarter. "...and twenty-five makes fifteen dollars..." Now you've eliminated the coin change: You're at fifteen dollars on your way to twenty. That one ought to be pretty easy, too. "...and five makes twenty." Now you've got $5.28 in your hand -- the correct change -- and you didn't have to subtract anything. And count it out to the customer the same way you just counted it: Don't just drop it in his hand like a hanky full of snot.

Try that a few hundred times, if you must, to convince yourself that it will work every time. If you can't master it, get out of the cashier business. And better take a grown-up with you when you go shopping for yourself, or you're going to be shortchanged every time. After all, you won't know any better, and maybe the cashiers you encounter will be smarter than you. The odds don't seem to be good, but it could happen.

How the hell are you ever going to know whether a two-pound bag at 89 cents is a better buy than a five-pound bag at $1.99? And Heaven help you if you encounter a 60-ounce package.

Maybe you'll just eat out. No, wait, that's where I came in.