It takes a runner 4 h, 3 min, 45 s to run a marathon. if the distance of a marathon is 42.2 km, what is the average speed of the runner?

You need to convert the 4 hr 7 min 29 s all to the same unit to start off with, the best thing to do usually is to convert to the smallest unit so we convert to seconds:

29s = 29s, no need to change anything there

7min = 7 X 60s = 420s

4h = 4 X 60min = 240min, 240 X 60s = 14,400s

Now that we've converted all the time units to the same unit (seconds), we add them together:

29 + 420 + 14,400 = 14,849s

Generally when we talk about speed and are using seconds as our unit for time, we express speed in m/s (metres per second), so now we convert the kilometres to metres:

1 km = 1000 m, so 42.2 km = 42.2 X 1000 = 42,200m

Finally, we can use our formula:

42,200 / 14,849 = 2.841942218 m/s

(I kind of rushed the calculations a little bit so that figure may not be 100% accurate, but the method is definitely correct)

Hope this helped :)