A man walks on a straight road from his home to a market 2.5 km away with a speed of 5 kmh-1. Finding the market closed, he instantly turns and walks back home with a speed of 7.5 kmh-1. The average speed of the man over the interval of time 0 to 50 min is equal to

Man walks from his home to market with a speed of 5 kmh-1. Distance = 2.5 km and time                              =dv=2.55=12=30 minand he returns back with speed of 7.5 kmh-1 in rest a time of 20 min.Distance =7.5×2060=2.5 kmSo, average speed = total distance  total time =(2.5+2.5)(50/60)h=305km/h