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M21-06

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M21-06  [#permalink]

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New post 16 Sep 2014, 01:10
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A
B
C
D
E

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  25% (medium)

Question Stats:

70% (01:32) correct 30% (01:37) wrong based on 141 sessions

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Trains leave station A for station B every 15 minutes starting 8:00 am. The distance between the stations is 80 km. If a train leaves station B for station A at 11:05 am, how many oncoming trains will it meet?


(1) Trains from station A to station B travel at 40 kmh

(2) The train from station B to station A travels at 40 kmh

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Re M21-06  [#permalink]

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New post 16 Sep 2014, 01:10
Official Solution:


Statement (1) by itself is insufficient. If the train from \(B\) to \(A\) is fast enough to cover 80 km in 5 minutes (theoretically), it will not meet any of the oncoming trains that depart from \(A\) after 11 am.

Statement (2) by itself is insufficient. If trains from \(A\) to \(B\) are too slow, the train from \(B\) to \(A\) might meet all the trains that departed from \(A\) since 8:00 am.

Statements (1) and (2) combined are sufficient. There is no unknown quantity left.


Answer: C
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Re: M21-06  [#permalink]

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New post 10 Sep 2016, 16:25
good question, need to focus on the first 05 minutes from 11 to 11.05 am, first train might cover the distance less than in 05 minutes,
if both train leaves at the same time, then B is the correct answer
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Re: M21-06  [#permalink]

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New post 08 Oct 2017, 02:47
Bunuel,

It is given that every 15 mins trains leave from A to B starting from 8 AM i.e. at 8 , 8:15,8:30 , 9, 9:15....11, 11:15

Now in the given the question, train from B to A left at 11:05 AM. Now question is about how many trains does B to A meet from A to B.

In the question, when it is mentioned that every 15 mins trains leave from A to B then even at 11 AM trains would have started...right ?..why this case is not considered.

Now in the official answer is C, can you please give the actual number of trains do B to A train meets ? Also full solution to clear the above.
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Re: M21-06  [#permalink]

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New post 21 Oct 2017, 08:00
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Until what time the trains run in both direction is not mentioned, how is it possible to calculate the number of train meeting.
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Re: M21-06  [#permalink]

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New post 21 Oct 2017, 08:12
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Pm091989 wrote:
Until what time the trains run in both direction is not mentioned, how is it possible to calculate the number of train meeting.


Hi Pm091989

Although time is not required here but if you want to calculate the time when trains meet you can do that

as trains are moving in opposite direction, relative speed=40+40= 80

first train from A starts at 8 a.m and travels till 11.05 i.e till 3h 5min when the first train from B leaves

so first train from station A would have traveled 40*3h5min

distance left =80-40*3h5min =x

the remaining distance will be covered at a relative speed of 80 km/h

hence time taken by 1st train from A to meet train from B = x/80+3h5min
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Re: M21-06  [#permalink]

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New post 13 Mar 2018, 17:43
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I mean, if the trains run on the route till 6:00 P.M. then it would be a different answer and if the trains run on the route till 10:00 P.M. the it would be a different answer.Hence, the answer should be E.
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Re: M21-06  [#permalink]

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New post 14 Mar 2018, 11:51
Pm091989 wrote:
I mean, if the trains run on the route till 6:00 P.M. then it would be a different answer and if the trains run on the route till 10:00 P.M. the it would be a different answer.Hence, the answer should be E.


Hi Pm091989

We know the distance between the stations and from both the statements we know the speeds of the trains. So I can calculate the travel time. Why should we consider different travel times here?
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Re: M21-06  [#permalink]

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New post 07 Apr 2019, 20:28
One thing I'm confused about this question is that wouldn't B be sufficient? The distance between the two stations is 80km, the train that moves from B to A is moving at 40km/hr. It means that the train will arrive at 1:05PM. Since the train leave leaves every 15 minutes, it means that it will meet all the trains that left between 11am to 1pm.
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Re: M21-06  [#permalink]

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New post 07 Apr 2019, 20:44
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clifflin3 wrote:
One thing I'm confused about this question is that wouldn't B be sufficient? The distance between the two stations is 80km, the train that moves from B to A is moving at 40km/hr. It means that the train will arrive at 1:05PM. Since the train leave leaves every 15 minutes, it means that it will meet all the trains that left between 11am to 1pm.


Consider one particular train which departs from A at 11am. If it can reach B before 11:05, then train from B won't meet it but if it won't then train from B will meet it.
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Re: M21-06  [#permalink]

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New post 07 Apr 2019, 21:06
Bunuel wrote:
clifflin3 wrote:
One thing I'm confused about this question is that wouldn't B be sufficient? The distance between the two stations is 80km, the train that moves from B to A is moving at 40km/hr. It means that the train will arrive at 1:05PM. Since the train leave leaves every 15 minutes, it means that it will meet all the trains that left between 11am to 1pm.


Consider one particular train which departs from A at 11am. If it can reach B before 11:05, then train from B won't meet it but if it won't then train from B will meet it.


I see. So if the question stem changed from 11:05 to 11am, then B would be sufficient then?
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Re: M21-06  [#permalink]

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New post 08 Apr 2019, 00:39
Bunuel wrote:
Trains leave station A for station B every 15 minutes starting 8:00 am. The distance between the stations is 80 km. If a train leaves station B for station A at 11:05 am, how many oncoming trains will it meet?


(1) Trains from station A to station B travel at 40 kmh

(2) The train from station B to station A travels at 40 kmh


given
15mins trains leave every time ; distance = 80 km
first train A to B 8 am and B to A ; 11:05
how many trains meet on journey
we need to find the speed of each train
#1
speed of A to B = 40 kmph so time ; 2 hrs it takes
insufficient as return speed not know
#2
same time; 2 hrs
insufficient
from 1 & 2
we can say
train leaving from 9:05 will meet trains starting from B to at 11:05
IMO C
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Re: M21-06   [#permalink] 08 Apr 2019, 00:39
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