I remember being at the store during a brief power outage once. I was a kid, don't remember what age but probably 8, +/- 1 year.
I had somehow come into the possession of a dollar, and decided to buy myself a large pack of gum.
The price was $.77 or something like that, and without the computer to do the math for her, the teenage cashier couldn't figure out the change.
I don't remember the exact amount she handed back to me, but I think it was over 80 cents. I took one look at it and said "This is too much."
I ended up having to tell her how much she owed me.
Sad when someone in high school can't do elementary school math, but I see it all the time now.