3 friends go to a restuarant to eat. Their bill comes to $25. They each have $10 bills so they give $30 to the waitress/waitor. The waitress/waitor puts $25 in the cash register, keeps $2 tip for herself and gives them each back 3 $1 bills. Each person takes $1. So now each person has paid $9. 9x3=27, the waitress has $2, this adds up to $29, so what happened to the $1 bill?