The buyer took goods worth 10 rubles from the seller and gave 25 rubles. The seller didn't have change, so he exchanged the money with a neighbor. After they settled and the buyer left, the neighbor discovered that the 25 rubles were counterfeit. The seller returned 25 rubles to the neighbor and began to think. What loss did the seller incur?

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Answer: 25 r. The loss consists in the fact that the seller gave 25 r. for fake 25 r; other exchanges can be disregarded.