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Question

A bus covers its onward journey with a constant speed of $$30km/h$$ and its return journey with a constant speed of $$60km/h$$. The average speed for its entire journey is 


A
90km/h
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B
45km/h
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C
40km/h
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D
15km/h
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Solution

The correct option is C $$40km/h$$
Lets consider, $$x=$$ distance covered by the bus
A bus covered $$x$$ distance with speed $$30km/h$$,
$$t_1=\dfrac{x}{30}hr$$

Bus returned to origin with  speed $$60km/h$$,
$$t_2=\dfrac{x}{60}hr$$

The average speed for the entire journey is
$$v_{avg}=\dfrac{Total\,    distance}{Total\,   time}$$ 
$$v_{avg}=\dfrac{2x}{t_1+t_2}$$
$$v_{avg}=\dfrac{2x}{\dfrac{x}{30}+\dfrac{x}{60}}$$
$$v_{avg}=\dfrac{2x}{0.05x}$$
$$v_{avg}=40km/h$$
The correct option is C.

Physics

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