Jenny borrowed $50 from Ken and $50 from Joan. She has $100 total. She bought a pants for %97. She recieved $3 as change. She returned $1 back to Ken, and now she owes Ken $9. She returns the other dollar back to Joan, and she owes Joan $49 too. Then, she kept the other dollar. So, $49 + $49 = $98. $98 + Jenny's dollar ($1) = $99.
Where did the other dollar go?
If you read and analyze it more, we solved the Math differently and wrong. It is like saying -1 - (-1) = 0.