Question

A boy runs for 10 min at a uniform speed of 9 km/h. At what speed should be run for the next 20 min so that the average speed comes to 12 km/hr?
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Answer 1

Answer:

Distance covered in 10 minutes = 9 kmph * 10/60 hrs = 1.5 km

Speed for the 2nd leg = V kmph

Distance coveed in 2nd leg = V * 20/60 = V / 3    km

Overall average speed = total distance / total time =

       ( 1.5 + V/3 ) /   (10+20)/60  = 12  km ph

  2 (1.5 + V/3) = 12

 1.5 + V/3 = 6

 V = 13.5 KMPH