Since Prashant is faster than Ashish he will take a lead and as they keep running the gap between them will also keep widening unlike on a straight track they would meet again even if Prashant is faster than Ashish.
Every second Prashant is taking a lead of
= [18×518−9×518] m =2.5 m over Ashish
Hence, he takes 5002.5=200s to take a lead of 500 m over Ashish.
Hence, they would meet for the first time after 200s.