You borrow $50 from mom and $50 from dad. You buy a shirt for $97 and get back $3 in change. You give $1 back to mom and $1 back to dad. You keep $1 for yourself.
Now you owe each of your parents $49.
$49+$49=$98 You still have $1. $98+$1=$99.
What happened to the other dollar?
