I'm not sure if that's what you mean, but I assume you might want to know how long did it take to make one lap if someone did 21 laps in 7 minutes.
If so, just divide 7 by 21:
7 ÷ 21 = 0.3333333... (repeating decimal) or 1/3 as a fraction.
It means that one lap took 1/3 of a minute (20 seconds).
If that's not what you meant, please specify the question :)