If you ran 15km per hour, for 20 minutes, how many distance would you cover

An hour is 60 minutes. If you ran for 20 minutes, you ran for 20/60 = 1/3 of an hour.

If you ran 1/3 of an hour, you also ran 1/3 of the distance:  (1/3) x (15)  =  5 km.