I've had a habit of making purchases by giving the cashier more than the required amount. This way I both get rid of loose change and don't accumulate more.
I'm always amazed that so many people can't do the math in their heads nor do they get it right even when their cash register does the math for them.
Worse than that is people that can't tell time on a standard clock. I used to help coach girls fast pitch softball travel teams. Some years ago the team I was with (18-under age group) was at a tournament and in the middle of one of the games one of the girls was standing behind me in the dugout and asked me what time it was. Since I was involved in the game strategy happening on the field I simply held up my arm and showed her the face of my watch. She looked at my watch, looked back at me and said, "I don't do well with that type of clock". Turns out she had digital clocks at home and had no clue what time was shown by the hands on my watch.