well we all know that
average speed = total distance traveled / total time of travel
thus total time of travel = total distance traveled / average speed
= [ 15 - 10 ] X 1000 / (6.02) 60
in the above equation we multiply the numerator by 1000 as the distances are in kilometers and the denominator by 60 as the time is to be reported in minutes
thus total time of travel = 13.8 minutes