We have been given that it takes 3.5 hours to complete 500-mile Nascar race.
We know that
1 hour = 60 minutes
and 1 hour = 3600 seconds.
Therefore, when we calculate the time taken in minutes we need to multiply 3.5 to 60 and to find the time to complete the race in seconds we need to multiply 3.5 by 3600.
Thus, the time taken to complete the race

Hence, it takes 210 minutes to complete the race.
Now, let us find the time in seconds. The number of seconds to complete the race is

Therefore, the order of magnitude of the number of seconds is 4.