What do you think?
The seller is selling a hat.
It costs 10 rubles. A buyer comes, tries it on, and agrees to buy it, but he only has a 25 ruble bill. The seller sends a boy with this 25 rubles to the neighbor to get change. The boy runs back and gives 10+10+5. The seller gives the hat and change of 15 rubles. After a while, the neighbor comes and says that the 25 rubles is fake and demands the money back. So what to do? The seller reaches into the cash register and returns the money to her. How much has the seller been deceived (including the cost of the hat)?
