What do you think?
The seller is selling a hat.
It costs 10 rubles. A buyer approaches, tries it on, and agrees to buy it, but he only has a 25 ruble note. 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 some time, the neighbor comes back and says that the 25 rubles are counterfeit, demanding her money back. So what to do? The seller reaches into the cash register and returns her the money. How much was the seller deceived (including the cost of the hat)?