At first glance, this classic riddle seems easy. A man steals a $100 bill from a shop, then returns and uses that same bill to buy $70 worth of merchandise. The shopkeeper accepts the bill and gives him $30 in change. But here’s the tricky question: How much money does the shopkeeper actually lose?

Some people say $200, while others argue it’s $130. Surprisingly, both answers are wrong. The correct answer is $100—and the reason becomes clear when you follow what the thief actually takes away.

Think about it this way: the stolen $100 bill eventually returns to the shopkeeper’s cash register as payment. But in exchange, the shopkeeper gives the thief $70 worth of merchandise and $30 in cash. Add those two amounts together, and you get $100.

$70 in merchandise + $30 in change = $100 in total losses.

So why do people get confused? The mistake is counting the original stolen $100 separately from the merchandise and change. That bill was stolen initially, but it came back to the shop during the purchase. Counting it again as an additional loss would mean counting the same value twice.

The real trick is to compare what the shopkeeper had before the incident with what remains afterward. The thief walks away with $100 worth of value, and the shopkeeper is left $100 worse off. Sometimes, the hardest part of a simple math problem isn’t doing the arithmetic—it’s knowing which numbers to count, and which ones not to count twice.