9.2. A student-entrepreneur bought several packs of masks from a pharmacy and sold them to classmates at a higher price, making a profit of 1000 rubles. With all the money earned, he again bought masks from the pharmacy (at the same price as the first time) and sold them to classmates (at the same price as the first time). This time, the profit was 1500 rubles. How much money did he spend on the first purchase?
Answer: 2000 rubles.

Solution 1: Let the package of masks in the pharmacy cost x rubles, and the student sold the masks for y rubles, and bought a packages of masks the first time. Then, according to the condition, $a(y-x)=1000$.

The revenue amounted to ay rubles, so the second time the student was able to buy $\frac{a y}{x}$ packages of masks. In this case, the profit amounted to $\frac{a y}{x} \cdot y-a y=\frac{a y(y-x)}{x}$ rubles. According to the condition, $\frac{a y(y-x)}{x}=1500$. From the two obtained equations, it follows that $\frac{1000 y}{x}=1500$, that is, $y=\frac{3}{2} x$. Substituting this result into the first equation, we get $a y-a x=a \frac{3}{2} x-a x=\frac{a x}{2}=1000$, from which $a x=2000$.

Solution 2: Let the student paid x rubles for the masks at the first purchase in the pharmacy. Then he sold them for $1000+x$ rubles. The second time he spent $x+1000+1500=x+2500$ rubles. Since the price ratio did not change, we form the proportion: $\frac{x}{x+1000}=\frac{x+1000}{x+2500}$. Solving the equation, we get $x=2000$.

## Criteria:

7 points - correct answer and complete correct solution;

6 points - correct solution with minor gaps or individual unclear places;

3 points - correct answer obtained based on reasoning with specific numerical data;

1 point - correct answer provided and only checked that it satisfies the condition;

only answer or answer with unclear calculations without explanations - 0 points.