it cannot be 150km/hr because if you judge it by distance, on the return trip you will not be on the road as long since your going faster and covering more distance in a shorter amount of time.....
Lets say its 150km distance @ 50km/hr then @150km/hr (return), so the first trip it will take you 3 hours and the return 1 hour. Total time, 4 hours, today distance traveled, 300km -----> 75km/hr average
Trick question....
Psalm 144(1): Blessed be the Lord my strength which teacheth my hands to war, and my fingers to fight.