Alexander B. answered • 09/05/17

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Moving at average speed 10km/hr, it will take 40/10 = 4 hours for entire ride.

Moving at the speed 20km/hr for the first 20 km took 20/20 = 1 hour. Therefore, (4-1)=3 hour left to complete the ride in order to fulfill the condition of average speed to be 10km/hr. Thus, the speed required for the last 20km is S= 20/3=6.67 km/hr (approx)