Making Change

I went into a convenience store and bought a soda. It cost me $1.07. I gave the lady $20.02. She gave me $19.00 in change. I was a nice guy so I looked at her with my hand still held out with the bills in it. She noticed my look and with a defensive tone she said "you gave me 3 pennies and I entered $20.03". I told her I only gave her 2 pennies and that she needs to take a dollar back and give me .95 cents. She repeated that I gave her 3 pennies while taking the dollar back and giving me my change. I left and wondered how people can work for a business and not know basic math to make change. This happens to me a lot at fast food joints where there are mainly high school students working. It still amazes me that they make wrong change but now I keep the extra money instead of correcting them because I've been f***** too many times in the drive through. So there.