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What is the spead of train?

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In a Tunnel 1 KM long. Two friends are standing at 600m inside the Tunnel (i.e. 600m from 1 side and 400m from other).Suddenly they heard the whistle of a train.Both started to run on opposite direction at spead of 10 KM/Hr.Both of them just survived.

What is the spead of train?

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In a Tunnel 1 KM long. Two friends are standing at 600m inside the Tunnel (i.e. 600m from 1 side and 400m from other).Suddenly they heard the whistle of a train.Both started to run on opposite direction at spead of 10 KM/Hr.Both of them just survived.

What is the spead of train?

the tunnel is 1km long so that when the first person reaches the on coming train they just miss it hence the person running in the same direction will have reached the same distance: 60m-40m=20m

Which is the same as 0.2km. divide this by speed they were running (10km/h)

0.2/10=0.02hr

Now this should be the same time that it takes the train to reach since they BOTH just survived and since it had to travel 1km

1/0.02=100/2=50km/h

QED (Quite Easily Done)

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the tunnel is 1km long so that when the first person reaches the on coming train they just miss it hence the person running in the same direction will have reached the same distance: 60m-40m=20m

Which is the same as 0.2km. divide this by speed they were running (10km/h)

0.2/10=0.02hr

Now this should be the same time that it takes the train to reach since they BOTH just survived and since it had to travel 1km

1/0.02=100/2=50km/h

QED (Quite Easily Done <!-- s:!: --><!-- s:!: --> )

is there a possibility that the train was stationary?

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the tunnel is 1km long so that when the first person reaches the on coming train they just miss it hence the person running in the same direction will have reached the same distance: 60m-40m=20m

Which is the same as 0.2km. divide this by speed they were running (10km/h)

0.2/10=0.02hr

Now this should be the same time that it takes the train to reach since they BOTH just survived and since it had to travel 1km

1/0.02=100/2=50km/h

QED (Quite Easily Done)

Schools taught me 1KM = 1000 m. Then, how can 20 m = .2 KM... Isnt it 0.02 KM???

If both survived, Isnt there a chance that conveys 'the moment the guy who ran 600m reached the end, the train entered in his side'???

This puzzle cannot be solved unless the trains position from the tunnel is given when it whistled...

Edited by rookie1ja

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Schools taught me 1KM = 1000 m. Then, how can 20 m = .2 KM... Isnt it 0.02 KM???

If both survived, Isnt there a chance that conveys 'the moment the guy who ran 600m reached the end, the train entered in his side'???

This puzzle cannot be solved unless the trains position from the tunnel is given when it whistled...

I think he meant 600m-400m = 200m, which IS .2 km

and also, it says they both just survived, which is how we know that the train entered RIGHT when the first person got out.

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50km/hr is right. basically if both parties were traveling at 10km/hr it would take the one person traveling 400m 2.4 minutes to reach the end of the tunnel and the person running 600m 3.6min to reach the end of the tunnel. Assuming the Train entered the tunnel at the 400m side the train would have to pass through the 1km tunnel in 1.2min to catch up to the second person. Thus traveling at a rate of 50km/hr. :)

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This puzzle cannot be solved unless the trains position from the tunnel is given when it whistled...

catchgeekay has found the flaw in the question.

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catchgeekay has found the flaw in the question.

Are you sure there's a flaw?

Hint: read the OP carefully. Then read post #8.

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See what you mean. There must only be one answer.

Edited by Mad Twit

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they start running the same speed, but in oposite directions. one goes 400m to get out and the other goes 600m. so the one that goes 400m gets out just as the train goes in on that side. the other one, after already going 400m, goes 200m in the same time it takes the train to go 1000m. which means the train is going five times as fast as the guy. so if he is going 10kph then the train must be going 50kph.

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There is another answer to this, which is dependant on how far away the train was when it was heard.

The main answer is based on the assumption that the tunnel is only just wide enough to take the train, which isn't stated. If the two people run in opposite directions towards the sides of the tunnel (and pin their backs against the wall) they can both just survive. The speed of the train is then determined by how far away it was when they heard it and how wide the train is.

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neida - I like that solution and considered it myself.

For those of you who think that the position of the train needs to be known in order to solve this puzzle I can tell you the train was 2 km away when it sounded it's whistle. Of course I can only say that after finding the train's speed (which was the solution to the original puzzle) and then working backwards to find the distance of the train from the tunnel. In other words, finding the answer to the original puzzle is not dependent on knowing the distance of the train from the tunnel.

nice work wahledor (post #10) and davasatoth (post #14) for explaining the solution so simply.

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I think technically at 50 kph, both were barely killed because at the point where they reach the end of the tunnel the train is occupying the same space. It's like a near miss, which means splat. So the answer x < 50 kph. Where the closer to 50 the train is, the closer the miss is. Then again, the boys could have been centered on the 600m mark in respects to the ends of the tunnel where their original distance from the center mark is how much the train missed them by at which point 50kph would miss, but it could have been an even closer miss at 50+ kph.

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For both to barely survive, following all need to be true:

1. Train will enter from direction in which the person has to run 400m

2. Train is at least (say) x km away from the tunnel mouth, which it can cover in 144 seconds i.e. time taken to run 400m @ 10kmph

3. Speed of train will be 5 times the speed of the 2nd person (=10kmph) as train needs to cover 1000m in time in which he covers 200m (400meter has been covered by him when train enters the tunnel and his friend is saved at other end)

This will lead to :

1. Speed of train : 50kmph

2. Distance from tunnel : 2km (144/3600 *50)

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neida - I like that solution and considered it myself.

For those of you who think that the position of the train needs to be known in order to solve this puzzle I can tell you the train was 2 km away when it sounded it's whistle. Of course I can only say that after finding the train's speed (which was the solution to the original puzzle) and then working backwards to find the distance of the train from the tunnel. In other words, finding the answer to the original puzzle is not dependent on knowing the distance of the train from the tunnel.

nice work wahledor (post #10) and davasatoth (post #14) for explaining the solution so simply.

You forgot to allow for the speed of sound. If the train is 2.4km away it from the men in would take around 7 seconds before they heard it and started running.

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You forgot to allow for the speed of sound. If the train is 2.4km away it from the men in would take around 7 seconds before they heard it and started running.

If you want to nit pick that much, your forgetting about the echos the whistle will cause in the tunnel.

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I think he meant 600m-400m = 200m, which IS .2 km

and also, it says they both just survived, which is how we know that the train entered RIGHT when the first person got out.

hence the correct answer is the train entered from first person side (400 m) at a speed of 50 km/hr. Right!!

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To put it another way, the first person made it the edge at T+144 seconds, while the second person made it at T+216 seconds, meaning the train traveled one kilometer in 216-144=72 seconds. (1[km]/72) * (60/1[m]) * (60[m]/1[h]) = 3600/72[km/h] = 50[kph]

Edited by OstermanA

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Hows about they ran to the walls and missed the train? therefore the speed wouldnt matter

as in if the train is ^ the tunnelwalls are | | and the people are < and >

|< ^ >| and the train misses them. it fits all of the requirements, so would this work?

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50 kmph ... and the spelling should be SPEED and not SPEAD

In a Tunnel 1 KM long. Two friends are standing at 600m inside the Tunnel (i.e. 600m from 1 side and 400m from other).Suddenly they heard the whistle of a train.Both started to run on opposite direction at spead of 10 KM/Hr.Both of them just survived.

What is the spead of train?

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I think technically at 50 kph, both were barely killed because at the point where they reach the end of the tunnel the train is occupying the same space. It's like a near miss, which means splat. So the answer x < 50 kph. Where the closer to 50 the train is, the closer the miss is. Then again, the boys could have been centered on the 600m mark in respects to the ends of the tunnel where their original distance from the center mark is how much the train missed them by at which point 50kph would miss, but it could have been an even closer miss at 50+ kph.

Good point and not really nit-picking. The speed must be something less than 50kph.

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'A' covers 400m @ 10 kmph in .4/10 = .04 hours

At this time the train enters the tunnel

'B' covers the extra 200m in .2/10 = .02 hours

At this time the train exits the tunnel

Time taken by train to travel 1 km = .02 hours

Therefore the speed of the train was 1/.02 = 50kmph

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