Question

a body Run for 10 minutes at a uniform speed of 9 km per hour at what speed should he Run for next 20 minutes so that the average speed comes to 12 km per hour

Answer 1

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/h

   2 (1.5 + V/3) = 12
  1.5 + V/3 = 6
  V = 13.5 Km/h