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Question

One car moving on a straight road cover one third of the distance with $$20\ km/hr$$ and the rest with $$60\ km/hr$$. The average speed of the car is


A
40 km/h
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B
80 km/h
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C
4623 km/h
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D
36 km/h
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Solution

The correct option is A $$36\ km/h$$
Let $$3d\ km$$ be total distance.
Time taken for first one third $$t_1 = \dfrac d{20}\ hr$$
Time taken for rest of journey  $$t_2 = \dfrac {2d}{60}\ hr$$
Total time   $$T =\dfrac { d }{ 20 } +\dfrac { 2d }{ 60 } =\dfrac { 5d }{ 60 } =\dfrac { d }{ 12 } $$
Average speed $$= \dfrac { 3d }{ \dfrac { d }{ 12 }  } =36\ km/hr$$

Physics

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