This being a college campus, I'd sort of assume that basic math skills would be relatively prevalent.
I just bought a coffee and a sandwich at a local shop, and there was a sign on the cash register saying "NEED $1 SINGLES PLEASE!" My total was $6.26. I looked through my cash, and I had a couple of twenties and a few singles, so I gave the girl a twenty, two singles, and a penny.
She took it all very uncertainly, and looked at me like I'd done something wrong. I just looked back at her expectantly. "Did you mean to give me two dollar bills?" "Yes, the sign says you need singles, I'm trying to help you out."
"But this won't...." she started. I said "just enter that in the cash register and you'll see what I mean."
When she did, and the $15.75 change amount displayed, she looked at me like I was a miracle worker, or something.