45 seconds is just 0.75 minutes. 21:45 is 21.75 minutes.
21.75 / 5 = 4.35 minutes per km
Or if you prefer to keep seconds, 21 / 5 = 4 minutes and 12 seconds, 45 / 5 = 9 seconds. Those are the times you’ve run 1 km in average and they add up to 4:21, 4:21/km.
45 seconds is just 0.75 minutes. 21:45 is 21.75 minutes.
21.75 / 5 = 4.35 minutes per km
Or if you prefer to keep seconds, 21 / 5 = 4 minutes and 12 seconds, 45 / 5 = 9 seconds. Those are the times you’ve run 1 km in average and they add up to 4:21, 4:21/km.