13.207. The express train travels the distance from Moscow to St. Petersburg 3 hours and 30 minutes faster than the passenger train, as it covers 35 km more in 1 hour. How many kilometers per hour does each of them travel, if the distance between Moscow and St. Petersburg is rounded to 650 km?
## Solution.

Let $x$ km/h be the speed of the passenger train, and $x+35$ km/h be the speed of the express train. According to the condition, $\frac{650}{x}-\frac{650}{x+35}=3.5$, from which $x=65$ km/h.

Answer: 65 and 100 km/h.