I want to buy some shoes that cost 50 and I borrow 25 from my father and 25 from my mother, when I get to the store the shoes now cost 45 I have 5 left, a friend lent her 3 and I have 2 left and I give her 1 and 1 to my parents and I owe them 24 each, but if 24 plus 24 is 48 plus 3 from my friend, why does she give me 51 and not 50?