100 km Tunnel

Algebra Level 1

There is a train which is 100 km in length and is going at 100km/h. The train is about to go through a tunnel which is exactly 100 km in length. How long is it, until the train completely passes through and comes out of the tunnel?

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1.5 hours 2.5 hours 4 hours 1 hour 2 hours

William Huang by William Huang , 17, Australia

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2 solutions

Mainak Physics
Jul 22, 2017

The tunnel length is 100km The length of the train is also 100km.
Speed of the train is 100km/h. (it means the train will travel 100kms in 1 hour).

Therefore, when the engine passes the tunnel, its 1 hour, but the train is itself also another 100 km long So, in this case the total time taken by the complete train to leave the tunnel = (length of tunnel + length of the train ). = (100 + 100) km. speed = 100 km/h. time taken = (200/100) = 2h.

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From, speed = distance time \mbox{speed}=\dfrac{\mbox{distance}}{\mbox{time}} , we have

100 = 100 time 100=\dfrac{100}{\mbox{time}}

time = 1 h o u r \mbox{time}=1~hour

However the train is 100 k m 100~km , so the total time is 1 + 1 = 1+1= 2 h o u r s \boxed{2~hours}

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