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.