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# Stations X and Y are connected by two separate, straight,

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VP
Joined: 26 Apr 2004
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Stations X and Y are connected by two separate, straight, [#permalink]  21 Jan 2005, 09:19
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Stations X and Y are connected by two separate, straight, parallel rail lines that are 250 miles long. Train P and train Q simultaneously left Station X and Station Y, respectively, and each train traveled to the otherâ€™s point of departure.
The two trains passed each other after traveling for 2 hours. When the two trains passed, which train was nearer to its destination?

(1) At the time when the two trains passed, train P had averaged a speed of 70 miles per hour.

(2) Train Q averaged a speed of 55 miles per hour for the entire trip.
Manager
Joined: 06 Sep 2004
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Only A is sufficient.

B is not because it qoutes the average speed of Q for the entire journey. We cannot calculate the average speed of B till the trains crossed each other.
VP
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ssumitsh wrote:
Only A is sufficient.

B is not because it qoutes the average speed of Q for the entire journey. We cannot calculate the average speed of B till the trains crossed each other.

Are we saying that, for a given avg speed, if a different ques asks that how many miles car X wud cover in 2 hrs, we can't ans that ques ??
Current Student
Joined: 28 Dec 2004
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banerjee

avg speed is given by, total distance over total time, the trains crossed well before the entire journey was over right? so for all we know, train Y went at 200mph for the first 30 minutes and then slowed down to maybe 10 mph for the remainder of the journey, giving an avg speed of 50mph...however, the question asks which train had travelled the furthest when the crossed each other, clearly its the train which had covered the most ground and we cant tell that by avg speed...
VP
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fresinha12 wrote:
banerjee

avg speed is given by, total distance over total time, the trains crossed well before the entire journey was over right? so for all we know, train Y went at 200mph for the first 30 minutes and then slowed down to maybe 10 mph for the remainder of the journey, giving an avg speed of 50mph...however, the question asks which train had travelled the furthest when the crossed each other, clearly its the train which had covered the most ground and we cant tell that by avg speed...

Not to be a pain....I know what u guys r saying, makes sense.....however, for any other ques where u r given avg speed and asked how many miles wud it cover in 2 hrs......u will just mupltiply speed * time for distance...right? Even here,....u don't really know what was the avg speed for those first 2 hrs ???
SVP
Joined: 03 Jan 2005
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If you are given the average speed in a time period, but asked about the total distance in a portion of that time period, you can't solve it.

A nice example is this:

Did John violate the 55mph speed limit?

I. John travelled 100 miles in 2 hours.
II. Somthing else.

I is not sufficient.
Director
Joined: 07 Jun 2004
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I am gogin with E on this as we do not know at what point did P and Q pass each other

what sis the OA
Director
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can you explain how you can get A
SVP
Joined: 03 Jan 2005
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Train P drove for 2 hours at average speed of 70, so distance is 140. Total distance is 200. It is closer to its destination. (60 miles away, vs train Q, 140 miles away.)
Intern
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RE [#permalink]  22 Jan 2005, 03:01
hello
Ans is D.
I think it is the relative speed concept.
When the two trains pass each other it is the
total distance/cumulative avg speed = time
here since in opp directions cumulative forms summation.
So answer has to be D.
Director
Joined: 21 Sep 2004
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speed * time= distance
70* 2 hrs= 140 ..
i.e 60 miles left..
but we dunno anything abt Q right.. how is it A?
Manager
Joined: 06 Dec 2004
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Ok so I am very confused on how you all are getting A.

From the question we know both are traveling a total of 250 miles and that they cross after 2 hours.

I) We know the speed of Train 1 is 70 MPH so from there we can calculate how far that train travled. 140 miles. But we know nothing about the speed of train 2. If train 2's speed was also 70 MPH then, at 2 hours they would cross and they EACH would have travled 140 miles and be equally close to their destination

II) Not sufficent becaues it is the average speed of the whole trip and we have no idea of its speed when they cross.

In combination we still down't know the speed of train 2 when they trains crossed

So I say E..... have we been given the OA, if so... what am i doing wrong here
SVP
Joined: 03 Jan 2005
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What do you think "cross" means? That means they were at the same point at the 2 hour point of time. If both trains have travelled 140 miles, considering the total miles (250) they would have crossed each other earlier than 2 hours, don't you think?
Manager
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HongHu wrote:
What do you think "cross" means? That means they were at the same point at the 2 hour point of time. If both trains have travelled 140 miles, considering the total miles (250) they would have crossed each other earlier than 2 hours, don't you think?

I have no idea.... that is why i am asking
SVP
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Well, "cross" means that they are at one point at the same time. That is what it means. Why are you not sure?
Manager
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No i understand what cross means.... I am just not sure why A is sufficent

unles we are saying based on the distance, and when the trains crossed we can tell how fast the second train was going thus could determine its distance and which was closer.....

Crossed is not what i have an issue with, its how A is sufficent.
Director
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I agree with thenine. Can somebody pleeeeeeeeeeeeeeeeeeeeeeeeaseeeeeee explain..
SVP
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Question is "When the two trains passed, which train was nearer to its destination?"

We know that when they passed each other, train P is 140 miles away from its origination and 110 miles (250-140) from its destination. Train Q is 110 miles from its origniation and 140 miles from its destination. So this information is sufficient for us to determine that train P is nearer to its destination.
SVP
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No problem.
VP
Joined: 26 Apr 2004
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I'm sorry for the late OA

The OA is A.

thanks
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