Problem 9.3. Traders Andrey and Boris bought 60 bags of potatoes each from the same farmer. All the bags cost the same.

Andrey sold all his bags, increasing their price by $100 \%$. Boris first increased the price by $60 \%$, and after selling 15 bags, increased the price by another $40 \%$ and sold the remaining 45 bags.

It turned out that Boris earned 1200 rubles more than Andrey. How many rubles did one bag of potatoes cost from the farmer?
Answer: 250.

Solution. Let the bag cost the farmer $x$ rubles. Andrei and Boris spent the same amount on buying 60 bags.

From the condition, it follows that Andrei sold 60 bags for $2 x$ rubles each, i.e., he received $60 \cdot 2 x$ rubles. Boris, on the other hand, sold 15 bags at a price of $1.6 x$ rubles and then sold 45 bags at a price of $1.6 x \cdot 1.4$ rubles. We get the equation

$$
2 x \cdot 60 + 1200 = 15 \cdot 1.6 x + 45 \cdot 1.6 x \cdot 1.4
$$

Solving it, we find $x=250$.