A man enters a shop and takes a $100 bill from the cash register without permission. Shortly afterward, he uses that same bill to purchase merchandise worth $70. The shopkeeper, unaware that the money was originally stolen, accepts the payment and returns $30 in change. How much money has the shopkeeper actually lost?
At first glance, this mathematical riddle can appear more complicated than expected. Because the same $100 bill appears in both the theft and the purchase, some people mistakenly add every amount mentioned in the story. This approach produces an incorrect result because it counts the same money more than once.
The correct answer is $100. After completing the transaction, the man leaves with $70 worth of merchandise and $30 in cash. Together, these amounts equal $100. Although the stolen bill returns to the register, it does not represent additional income because it already belonged to the business before the incident.
The simplest way to understand this puzzle is to examine what the shopkeeper ultimately loses. The merchandise is gone, and so is the $30 given as change. The original bill has been returned, meaning it should not be counted again. This entertaining brain teaser demonstrates how carefully following money through a transaction can prevent common calculation mistakes.