You see a shirt for £97 but you don't have the money, so you borrow £50 from your Dad and £50 from your mum so £50 £50=£100 right?. So now you have your £100 you go and buy your nice new shirt and get £3 change, You give £1 to your Mum and £1 to your Dad and you keep £1 yourself.
Now you owe them both £49.
£49 £49 your £1=£99
WHERES THE OTHER £1 ?