From the condition, Bahir covered 500 m at the same time as Arav covered 400 m. So, their speeds ratio is= = = . It means that when Bahir will cover 5 lapses, Arav will cover 4 lapses, so they will be at the starting point simultaneously at this time moment. It gives you the ANSWER to the problem's question: Arav will complete 4 lapses, when Bahir reaches him from behind on the track, completing his (Bahir's) 5-th laps.