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2014-08-09T10:20:07+05:30

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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

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2014-08-09T13:15:14+05:30

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So t = 60 min=1/6 hr
speed = 9 km/h
displacement = 9 x 1/6 = 1.5 km
again 
let speed = X km/h
time = 20 min = 20 / 60 = 1/3 hr
displacement = X x 1/3 = X/3
so av. velocity = total displacement /time taken

(15/10 +X/3)/(1/2) = 12
⇒x = 13.5 km



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