when 乙出發
甲 is at 100公里 and 乙 is at 0公里.
1 hour later,
甲 is at 200公里 and 乙 is at 50公里.
2 hours later,
甲 is at 300公里 and 乙 is at 100公里.
3 hours later,
甲 is at 400公里( which is the starting point = 0公里) and 乙 is at 150公里.
4 hours later,
甲 is at 100公里 and 乙 is at 200公里.
5 hours later:
甲 is at 200公里 and 乙 is at 250公里.
6 hours later,
甲 is at 300公里 and 乙 is at 300公里.