I am a lecturer in mathematics and I prepare a student for EGE.

Chocolate is 35 rubles. In the store, when buying 2 chocolates, the third one as a gift. How much chocolate can you get for 250 rubles?

Answer: 9 ( the correct 10)

On my question why 9, I got the answer that taking the tenth is not profitable and she would not buy it. How economical a child will be in the future :)