There is no missing dollar. Each woman paid $10 for their $30 bill. The actual bill was $25. When the waiter pocketed $2 and returned $1 to each woman, the bill essentially became $27. The $2 in the waiter’s pocket is included in the bill because the women had no idea it was being returned to them. Each woman DID pay $9 each. 9*3=27. The $29 from adding $2 to the bill should not be accounted for, since it was already secretly added to the bill the moment the waiter put it in his pocket.