Math Isn't My Friend

I just had to share. I picked up a piping hot on the way home from work. My total was $7.95, so I gave the lady $10.05 thinking I'd get $2 back. Here's my reasoning; I gave the lady a nickel, so that would have made it $8, meaning I'd get $2 back. I'm a fucking moron. The worst part is Dr. Dork tried explaining it to me, and I just don't understand.

That is all.