You see a shirt for 97 dollars.
You don't have the cash so you borrow 50 bucks from your mom and 50 bucks from your dad,
50+50=100.
So you buy the shirt and you have 3 dollars change.
You give your dad 1 dollar and your mom 1 dollar, and you keep 1 dollar.
Now you owe your mom 49 dollars and your dad 49 dollars. 49+49=98 + your 1 dollar=99 dollars.
Where is the missing 1 dollar?