It's a trick. That last addition is kind of meaningless. It's what you owe + what you still have in your pocket, which equals what? Not what you borrowed.
EDIT: They're just trying to confuse you by throwing in 2 lenders. Make it one. You borrow $100 from Dad, buy a $97 shirt (geez, really?) and get $3 back. You pay him back $2. So what? He has $2 of the original hundred, and you have $98 ($1 + $97 shirt). That's the same $98 you owe him. You can add your $1 to this and get $99, but that is about as meaningful as adding an armadillo to a banana.