You see a£97 top but don't have enough money so u borrow£50 from ur mumsy and £50 from ur dad=100 you buy the top and get £3 change then u decide to give ur mum £1 and ur dada £1 and keep the other £1 for ur self you now owe ur mumsy £49 and ur dad £49 so 49+49=98 and ur £1 makes £99 where is the missing £1?