[ Arithmetic. Mental calculation, etc.]

A customer walks into a store, picks out an item costing 20 rubles, and gives the seller a hundred-ruble note. The seller looks - no change. He goes to the neighboring department, gets the hundred-ruble note changed. He gives the customer the item and the change. The customer leaves. Suddenly, the seller from the neighboring department comes over, bringing the hundred-ruble note. It's a counterfeit! Our seller gives him his own hundred-ruble note. How much did our poor seller lose in the end?
Note: The seller would not have suffered any loss if it were not for the fake 100-ruble note.

## Solution

The seller gave the buyer goods and change totaling 100 rubles, and also paid 100 rubles to the second seller, but he had already received 100 rubles from the second seller. So the total loss is 100 rubles. It can also be solved differently. The buyer, in fact, "shortchanged" the seller by 100 rubles. It is on these 100 rubles that the seller lost.

## Answer

The seller lost 100 rubles.