7.5. Seryozha and Vasya set out towards each other at the same time: the first from point $C$, and the second from point $B$, the distance between which is 21 km. Seryozha's dog Bobik starts moving from point $C$ together with Seryozha and runs towards Vasya. Upon meeting him, without taking a breath, he rushes back to meet Seryozha, after which he turns around again and runs towards Vasya, and so on. They all move along the same road, and the speed of Seryozha is 4 km/hour, Vasya's speed is 3 km/hour, and Bobik's speed is 11 km/hour. What is the total distance (back and forth) that Bobik ran? Justify your answer.
Solution: The approach speed of the pedestrians is $3+4=7$ km/h, and they will meet after $21: 7=3$ hours from the start of their movement. Continuously running, Bobik will cover $3 \cdot 11=33$ km during this time.

Answer: 33 km.

Recommendations for checking:

| is in the work | points |
| :--- | :--- |
| Correct and justified answer | 7 points |
| A correct approach to the solution resulted in an incorrect answer due to  arithmetic errors | 6 points |
| Correctly identified and justified the pattern by which  Bobik's path changes on each segment: from Seryozha  to Valera and back | 4 points |
| Correctly calculated Bobik's path on the first few  segments: from Seryozha to Valera, from Valera to Seryozha, etc. | 2 points |
| Calculations that do not show the ideas of the solution and/or  illustration of the statement with specific examples | 0 points |