I wasn't actually saying anything about her intelligence; I regularly get tangled up in arithmetic simply trying to calculate the tip, and that's not at the end of a lunch rush. Also I'm not very good at making sandwiches, let alone wrapping them up so they won't leak.

Anyway, my questions weren't actually rhetorical: I really was trying to figure out whether what I was doing was strange and/or pointless. And I think I've decided that carrying a $5 bill instead of five singles is a pretty trivial thing to care about, and not worth the potential social friction caused by cleverly trying to arrange it.