9.1. A merchant bought several bags of salt in Tver and sold them in Moscow with a profit of 100 rubles. With all the money earned, he again bought salt in Tver (at the Tver price) and sold it in Moscow (at the Moscow price). This time the profit was 120 rubles. How much money did he spend on the first purchase?
Answer: 500 rubles.

Solution: The additional 100 rubles spent the second time brought the merchant an additional 20 rubles in profit. Therefore, the first time, to earn $5 \cdot 20=100$ rubles in profit, the merchant must have paid $5 \cdot 100=500$ rubles.

Second solution: Let the amount of the first purchase be $x$, then for every ruble invested in Tver, he would receive $\frac{x+100}{x}$ rubles in Moscow. Consequently, after the second purchase-sale, he would receive $\frac{x+100}{x}(x+100)=x+100+120$ rubles. Solving this, we get $x=500$.

Grading criteria: Only the answer with verification: 2 points.