A man enters a shop and secretly takes a $100 bill from the cash register without permission. Shortly afterward, he uses that same bill to purchase $70 worth of merchandise. The shopkeeper, unaware that the money was stolen, accepts the payment and gives the man $30 in change.

At first glance, this puzzle may seem confusing. Some people try to add every amount mentioned, counting the stolen $100, the $70 in merchandise, and the $30 in change separately. However, this approach counts the same money more than once.

The correct answer is $100. To understand why, consider what the shopkeeper actually loses during the entire incident.

First, the man steals $100 from the register. This means the shopkeeper temporarily loses $100 in cash.

Next, the thief returns that same $100 bill when purchasing the merchandise. The shopkeeper unknowingly accepts the stolen money as payment and gives the man $30 in change.

After the transaction, the original $100 bill is back in the register. However, the shopkeeper has lost merchandise worth $70 and paid out $30 in cash.

Adding these losses together gives $70 worth of merchandise plus $30 in change, totaling $100. The stolen bill itself should not be counted again because it has returned to the shopkeeper.

Therefore, the shopkeeper’s total loss is $100, consisting of $70 in merchandise and $30 in cash. This classic brain teaser demonstrates how following the money carefully can help avoid double-counting and reveal the correct answer.