You saw a shirt for 97$.
You don't have the cash. So you borrowed 50$ from your mom and 50$ from your Dad.
50 + 50= 100
you bought the shirt, and had 3$ change.
You give your mom a 1$ and your dad 1$ and keep the other dollar for yourself.
You owe your mom 49$ and your dad 49$.
49 + 49= 98+ your 1$= $99
Where is the missing $1?