Problem 773

Answer: 100 rubles. Solution. Let there be $x$ watermelons, then there will be $150-x$ melons. If a container can hold 120 melons, then one melon occupies $\frac{1}{120}$ of the container. Similarly, one watermelon occupies $\frac{1}{160}$ of the container. Therefore, if the container is fully loaded with 150 watermelons and melons (and the container is full), then $\frac{x}{160}+\frac{150-x}{120}=1$. From this, $x=120$. So, there were 120 watermelons and 30 melons. If a watermelon costs $a$ rubles, and a melon costs $b$ rubles, then (since the cost of watermelons and melons is the same): $120 a=30 b=12000$. Therefore, $a=100, b=400$.