you see a £97 shirt, but don,t have any cash so you borrow £50 from your mom and £50 from your dad = £100. you buy the shirt, have £3 change so give mom £1, dad £1 and keep the other £1 for yourself. you now owe mom £49, dad £49 so £49 + £49 = £98 & your £1 makes £99 so where is the missing £1 ?