1) the total distance of the nth meeting is (2n-1) *1000 m.

The speed sum is 3.5m/s.

Second meeting time: 3000/3.5=6000/7 seconds.

The third meeting time: 5000/3.5= 10000/7 seconds.

The fourth meeting time: 7000/3.5=2000 seconds.

2) The total distance between the n th meeting and the n+ 1 meeting is 2000 m.

Time interval: 2000/3.5=4000/7 seconds.

Hope to adopt.