7.1 In a bookstore, Vasya and Tolya were interested in the same book. Vasya was short of 150 rubles to buy it, and Tolya was short of 200 rubles. When Vasya asked Tolya to lend him half of his cash, Vasya was able to buy the book and still had 100 rubles left for transportation. How much did the book cost?
Answer: 700 rubles. Solution: If $x$ is the price of the book, then Vasya had $(x-150)$ rubles, and Petya had $(x-200)$ rubles. By setting up the equation $x-150+\frac{x-200}{2}=x+100$ and solving it, we find $x=700$.