The Case of the Bogus $20 Bill
A customer goes into a shoe store and buys a pair of shoes for $5, but pays with a counterfeit $20 bill. The shoe store owner does not realize that the $20 bill is bogus. However, the shoe store owner notices that she does not have change for the $20 bill - so she runs to the grocer next door. The grocer gives the shoe store owner four $5 bills for the bogus $20 bill. The shoe store owner returns to her store and gives the customer the shoes and $15 change. Later the grocer arrives at the shoe store with the FBI to inform the owner that the $20 bill was bogus! So the shoe store owner is forced to give the grocer $20, and the FBI keeps the bogus bill. How much did the shoe store owner lose?