1. The distance between cities $A$ and $B$ is 435 km. A train departed from $A$ at a speed of 45 km/h. After 40 minutes, another train departed from city $B$ towards it at a speed of 55 km/h. How far apart will they be one hour before they meet?
Answer: The trains are approaching each other at a speed of 100 km/h. Therefore, in the last hour, they will together cover 100 km. This is the distance that will be between them one hour before the meeting.

Evaluation. 7 points for the correct solution.