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Question

A person goes from A to B at a speed of 40 m s−1 and comes back from B to A at a speed of 60 m s−1. What is his average speed?

A
34 m s1
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B
42 m s1
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C
40 m s1
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D
48 m s1
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Solution

The correct option is D 48 m s1
We know that,
Average speed=Total distance travelledTotal time taken

Let the distance between A and B be d.
The total distance travelled =2d.
Let the time taken to travel from A to B be t1 and the time taken to travel from B to A be t2

i) Speed from A to B =dt1=40
t1=d40
ii) Speed from B to A =dt2=60
t1=d60

Now, the required average speed =2dt1+t2

=2dd40+d60

=2d(40+60)×d40×60

=2×2400100=48 m s1

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