Not that Smart

"Is that right?" Katie calls down the hall as I walk to the kitchen, but I already have the nagging suspicion that my math is off. I was talking about the fact that my boss over-tipped at an event today, thinking how smart I was that I noticed.

But now I realize that I totally got it wrong, and relief floods me that I didn't say anything. 

I sometimes think I'm smarter than I actually am.