Not To Subtract From The Situation

(I am waiting in line at the check out. The customer ahead of me has just bagged their groceries.)

Cashier: “Your total is $66.14, sir.”

Customer: “I want to pay with this $100 bill, but instead of giving me change, can you put the rest on a gift card?”

Cashier: “Sure, I can do that.”

(The cashier stares at the register for a moment.)

Cashier: “Um… do you know how much that’s going to be on the gift card? I can’t find out how much your change is until you pay, and I still have to add your gift card.”

Customer: “I don’t know; just whatever is left out of $100.”

Cashier: “Let me see if there’s a calculator around…”

Me: “It’s $33.86.”

(The cashier and customer look at me uncertainly.)

Cashier: “Are you sure?”

Me: “Yes, just put it in; you’ll see.”

Cashier: “…okay.”

(The cashier puts it in hesitantly.)

Cashier: “…hey, you were right! How’d you do that?”

Me: “I’m a teacher. I subtract from 100 every time I grade papers.”

Customer: “But you did that in your head without a calculator or anything.”

Me: “Yeah, I actually teach math, so I have to be able to do math.”

Cashier: “Wow you must have to be really smart to teach math like that!”

Me: “Um…”