A man steals a $100 bill directly from a store’s cash register. At that moment, the store is already down $100 in cash. But then the man does something that makes the puzzle seem more complicated: he comes back and uses that same stolen bill to buy $70 worth of merchandise.

The cashier accepts the $100 bill and places it back into the register. That means the original stolen $100 has now returned to the store, so that part of the loss has effectively been canceled out. However, the store still has to give the man $30 in change.

On top of that, the man walks away with $70 worth of goods. So the store has lost $30 in cash plus $70 in merchandise. Those two amounts add up to exactly $100.

The common mistake is to add the original $100 theft to the $70 in goods and the $30 in change, which would produce $200. But that counts the same $100 bill twice, because the stolen bill was returned when the man used it to make the purchase.

So the correct answer is $100. The store ends up losing $70 worth of merchandise and $30 in cash, for a total loss of $100.