You want to buy candy at the store for 7 dollars, you borrow 5 dollars from your dad and 5 dollars from your mom. You buy your candy, and the cashier gives you 3 dollars in change. You need to pay your parents back, so you give one dollar to your mom, one dollar to your dad, and you keep the last dollar. Now you owe each parent 4 dollars. 4 plus 4 is 8, plus the one dollar you kept makes 9, but you got 10 dollars from both your parents. Where's the missing dollar?
LoL, you are missing the logic here. First you have 10, minus 7 for the candy. You now have 3, you return 2 dollars to your parents therefore your debt now is 8. If you pay them 1 dollar, then your debt will be 7 which you used to buy candy.
You owe your parents 10 dollars. If you give each parent a dollar, you now owe your parents 8 dollars. 8 plus 1 is 9, that's true, but 9 dollars is not how much you owe your parents. Recall that you owe your parents 8 dollars. Use the dollar that you kept from earlier to help pay your debt to them. 8 minus 1 is 7, and 7 dollars is how much your candy cost you. You could return the candy and get 7 dollars to pay off your debt, or you could just wait until your parents forget about it.