The scammed storeowner
A man visits an electronics store and buys a pair of hi-fi speakers for €80. He pays the storeowner a €100 note. The storeowner doesn't have any change. He therefore approaches the neighboring storeowner and asks him for change.
The neighboring storeowner takes the €100 note and gives ten €10 notes to the storeowner. The storeowner then keeps eight of the notes with him and returns two notes to the man. The man walks away with the pair of hi-fi speakers and €20.
After some time, the neighboring storeowner finds out that the €100 note is fake and asks the storeowner for the note to be replaced. The shopkeeper with no other alternative gives his neighbor another €100 note. What is the loss suffered by the storeowner if his markup on the pair of hi-fi speakers is 60%?