you see a shirt for £97, but dont have any money so borrow £50 from mum & £50 from dad = £100. you buy the shirt and have £3 change, so give mum £1, dad £1 and keep £1 for yourself. you now owe mum £49 and dad £49 so £49 + £49 = £98 & your £1 = £99 so where's the missing £1?