A train travels from Nagpur to Chennai with a constant speed of $80 k m p h$ and returns from Chennai to Nagpur with a constant speed of $60 k m p h$ . Find out the average speed of the train.

$68 k m p h$

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$68.5 k m p h$

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$78 k m p h$

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$78.5 k m p h$

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Solution

The correct option is B
$68.5 k m p h$

Let the distance between Nagpur and Chennai be d km.

For the first half of journey

Time $= T_{1} = \frac{d}{V_{1}} = \frac{d}{80} h$

For the second half of journey

Time $= T_{2} = \frac{d}{V_{2}} = \frac{d}{60} h$

Total time $= T_{1} + T_{2} = \frac{d}{80} + \frac{d}{60} = \frac{7 d}{240} h$

Total distance = d + d = 2d

Average speed $= \frac{total distance}{total time}$

$= \frac{2 d}{\frac{7 d}{240}} = \frac{240 \times 2}{7}$

$= 68.5 k m p h$

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A train travels from Nagpur to Chennai with a constant speed of 80 kmph and returns from Chennai to Nagpur with a constant speed of 60 kmph. Find out the average speed of the train.

A train travels from Nagpur to Chennai with a constant speed of $80 k m p h$ and returns from Chennai to Nagpur with a constant speed of $60 k m p h$ . Find out the average speed of the train.