A man steals a $100 bill from a shop and later returns to the same store with that exact bill. He uses it to purchase $70 worth of goods, and the shop owner gives him $30 in change. At first, it can seem as though the store has somehow lost far more than $100 because the stolen money was involved twice.

The easiest way to solve it is to follow what physically leaves the shop. When the man initially steals the $100 bill, the shop is temporarily down $100 in cash. But when he comes back and spends that same stolen bill, the $100 returns to the cash register.

The shop owner then gives the man $70 worth of merchandise. That merchandise permanently leaves the store with him, so the shop loses $70 in goods.

The owner also gives the man $30 in change. That $30 permanently leaves the store as well. The stolen $100 bill itself is back where it originally came from, so it is no longer an additional loss.

Therefore, the shop owner’s total loss is $100: $70 worth of goods plus $30 in cash. The trick is not to count the stolen $100 twice.