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M21-06

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M21-06 [#permalink]

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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
[Reveal] Spoiler: OA

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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, 15:25
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, 01:47
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, 07:00
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, 07:12
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] 21 Oct 2017, 07:12
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