challenge of motion .
Classical Mechanics
Level
1
A boy runs for 10 minute at a uniform speed of 9 km/h. At what speed should he run for the next 20 minute so that the average speed comes to 12 km/h ?
10.2 km/h
13.5 km/h
7.72 km/h
8.2 km/h
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1 solution
left over distance = 12 X 30/60 - 9 X 10/60 = 4.5 km so appropriate speed = 4.5 X 60/20 = 13.5 km/hrs