Simple but interesting task

This problem for a second-class church parish school was invented by Leo Tolstoy. Who can solve it correctly only 30% of high school students, only 20% of university students and 10% of employees of banks and credit institutions. Every time I open the task and every time I forget the correct answer. From this, it is true, it only gets more interesting.

The seller sells a cap, which is worth 10 p. Suitable buyer, it measures and agree to take, but it has only 25 p. The seller sends the boy with these 25 p. to a neighbor, to exchange. Boy resorts and gives 10 + 10 + 5. Seller pays cap and commissioning of 15 rubles. After a while the neighbor comes and says that 25 p. fake calls to give her money. What to do? A guy climbs to the cashier and return her money.

Q: How much deceived seller?


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