Ah, today's kids!
When I was in college, I worked the concession stands at the games. Had to add all those hot dogs, chips, drinks, in my head! No cash register, just a cash box.
And yes, I counted the change for the customers just like you.
Today's kids don't have to do do this because all cash registers are computerized. When I hand a cashier $5 and one nickel for a bill of $4.05, they can't figure that one out! Yes, even the kids in advanced classes. They can do more than we ever could (I didn't use a calculator till college statistics) but WE can also do things they can't.