Given data:
Yan speed;

Christopher speed;

Christophe starts 30 s later than Yan. Therefore, Christophe takes 30 s less than Yan to reach the same distance.
Part (A)
The distance is given as,

Let both Yan and Christophe meet at d distance from the start position. Therefore,

Substituting all known values,

Therefore, 600 s after Yan's departure Christophe will join him.
Part (B)
The distance is given as,

Substituting all known values,

Therefore, Christophe joins Yan after 6.17 km from the start.
The star that would be hotter would be; the star emitting visible light.
Because they are larger planets the gravitational pull is heavier and they attract more celestial bodies
ANOTHER RUNNING DOG
Explanation:
In the given question it is to find a suitable reference point to describe the motion of dog. Here I could suggest that it is better to compare the dog with another running dog to create the relative speed difference to get a reliable motion variation.
Because the motion of dog is in the linear with respect to the another dog and to the acceleration produced by the dog in the required interval is easy to calculate with respect to another dog which is already in motion.
Hence, I suggest that Motion of dog can be analysed better by analyse the motion variation of dog with another dog running.
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