[ Problems on percentages and ratios ]

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?

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The additional 100 rubles spent for the second time brought the merchant an additional 20 rubles in profit. This means that the first time, to get $5 \cdot 20=100$ rubles in profit, the merchant had to pay $5 \cdot 100=500$ rubles.

## Answer

500 rubles.