When Devi passed Kumar, there was still 6 minutes before he would pass Li Ting.
In that 6 minutes, he would have travelled 100m/min × 6min = 600m
In that same 6 minutes, Li Ting would have travelled 75m/min × 6min = 450m.
Total distance the two of them travelled in 6 min = 600m + 450m = 1050m. Then they passed each other.
This tells us that when Devi passed Kumar, Devi and Li Ting were still 1050m apart. This also means that when Devi passed Kumar, Kumar was 1050m ahead of Li Ting.
Difference in speed of Kumar and Li Ting = 80m/min - 75m/min = 5m/min
Since he is 5m ahead of Li Ting every minute, amount of time they had been travelling when Kumar passed Devi = 1050m ÷ 5m/min = 210min
Distance travelled by Devi and Kumar in 210min = 210min × (100m/min + 80min/min)
= 210min × 180m/min = 37800m = 37.8km
Since they both started from opposite towns at the same time, then in 210min, they covered the entire distance between the towns combined, and passed each other.
So the distance between the towns is 37.8km.