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Question

A train travels from city A to city B with a constant speed of 10 ms1 and return back to city A with a constant speed of 20 ms1. Find its average speed during its entire journey.


A

25 m/s

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B

403 m/s

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C

203 m/s

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D

20 m/s

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Solution

The correct option is B

403 m/s


Average speed is always calculated from the ratio of total distance to the total time taken. Average speed is not the arithmetic average of speeds in a journey.
Consider, the distance between the two cities A and B = x m.
Time taken by the train to travel from A to B =x10=t1 (say)
Time taken to come back from B to A =x20=t2 (say)
Average speed =Total distanceTotal time=x+xt1+t2=2xx10+x20
=403 m/s


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