Three people are paying the bill, it's ?30, split 3 ways. ?10 each. ?30 Total After paying, the waiter returns and says he over charged by ?5. Dave says "Thanks, keep ?2, just give us a pound each" The waiter pockets the ?2 and gives each diner ?1. Thus they paid ?9 each. So, ?9 each, 3 x 9 = ?27, the waiter kept ?2 27 + 2 = ?29 Where is the missing pound?