Puzzles/Non-scientific puzzles/Boy, Girl, and a Dog/Solution

Since the dog travels at 10 km/hr, in 1 hour it will cover 10 km.

The boy moves at 5km/hr the girl at 4km/hr. After 1 hour the distance between them is 1km. The dog is within 1km. The distance between the boy and girl will increase linearly over the one hour, to its maximum of 1km. During this time the dog has been running back and forth at 10km/hr.To calculate its exact position is a VERY scientific question.

The boy moves at 5km/hr the girl at 4km/hr, but we don´t know for how long they have being moving. They could have just started to move and there´s virtually no distance between them. So the dog, whatever his speed, could just had travel a couple meters. Also we dont know if he stops when he gets back to the boy or continues running, and for how long. Or if his speed is constant and movement linear. Maybe after a 100 meters he got tired and took a nap.