Mr Wang sells shoe.
A pair of shoe costs $20 and its selling price is $30.
A customer bought a pair of shoe and paid with a $50 note.
Mr Wang didn't have change so he went to his neighbor to break it into small notes.
He returned $20 to the customer.
The next day, Mr Wang's neighbor realized that the $50 note was a fake.
Mr Wang compensated his neighbor $50.
How much money did Mr Wang lose?
Mr. Wang lost $40 ($20 shoe and $20 returned to customer)