Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized for You

we will pick new questions that match your level based on your Timer History

Track Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice Pays

we will pick new questions that match your level based on your Timer History

Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.

It appears that you are browsing the GMAT Club forum unregistered!

Signing up is free, quick, and confidential.
Join other 500,000 members and get the full benefits of GMAT Club

Registration gives you:

Tests

Take 11 tests and quizzes from GMAT Club and leading GMAT prep companies such as Manhattan GMAT,
Knewton, and others. All are free for GMAT Club members.

Applicant Stats

View detailed applicant stats such as GPA, GMAT score, work experience, location, application
status, and more

Books/Downloads

Download thousands of study notes,
question collections, GMAT Club’s
Grammar and Math books.
All are free!

Thank you for using the timer!
We noticed you are actually not timing your practice. Click the START button first next time you use the timer.
There are many benefits to timing your practice, including:

Two trains of length 100m and 250m run on parallel lines [#permalink]

Show Tags

29 Sep 2009, 10:55

2

This post received KUDOS

5

This post was BOOKMARKED

00:00

A

B

C

D

E

Difficulty:

(N/A)

Question Stats:

54% (02:55) correct
46% (01:26) wrong based on 25 sessions

HideShow timer Statistics

Two trains of length 100m and 250m run on parallel lines. when they run in the same direction it will take 70 sec to cross each other and when they run in opposite direction, they take 10 sec to cross each other. find the speed of faster train.

Let the faster train run at x m/s and the slower train run at y m/s.

The first statement states that it will take 70 seconds to cross each other running in the same direction. The first important thing her is that the faster train is behind the slower train, otherwise they would never cross. Secondly, in order to cross the slower train, the faster train will need to cover 350m MORE (250 m + 100 m) in the 70 seconds than the other train.

\(\frac{350}{(x - y)} = 70\)

\(\frac{5}{(x - y)} = 1\)

\(5 = x - y\) (Equation 1)

The second statement states that it will take 10 seconds to cross each other running in the opposite direction. The difference between this and running in the same direction, is that the slower train's speed CONTRIBUTES to the cross rather than inhibiting it. If the slower train was standing still, the faster train would take (350/x) seconds to pass it. Since the slower train is moving in the other direction though, it will take (350/ x+y) seconds to cross.

\(\frac{350}{(x + y)} = 10\)

\(\frac{35}{(x + y)} = 1\)

\(35 = x + y\) (Equation 2)

Solving these two equations, you get x = 20 m/s, and y = 15 m/s.

Re: Two trains of length 100m and 250m run on parallel lines [#permalink]

Show Tags

28 Mar 2012, 21:58

Hello,

Though the explanation to the given problem is simple, could someone explain with example, as I still find it difficult to intrepret, which train is 100m and which one is 250 m long?

Two trains of length 100m and 250m run on parallel lines. when they run in the same direction it will take 70 sec to cross each other and when they run in opposite direction, they take 10 sec to cross each other. find the speed of faster train.

Alternative approach:

To cross each other (either in same or opposite direction), the trains have to cover a distance of 250 + 100 = 350 m (the faster train should cover the entire slower train and then its own length so that they completely cross each other)

When they run in the same direction, they cover this distance in 70 sec. So their relative speed in this case (which is the difference in their speeds) is 350/70 = 5 m/s When they run in opposite directions, they cover this distance in 10 sec. So their relative speed in this case (which is the sum of their speeds) is 350/10 = 35 m/s

If sum if 35 and difference is 5, you should quickly jump to 20 and 15.

Note: We used the concept of relative speed here. When 2 objects move in same direction, their relative speed i.e. speed relative to each other is the difference of their speeds. When the 2 objects move in opposite directions, their relative speed i.e. speed relative to each other is the sum of their speeds.
_________________

Though the explanation to the given problem is simple, could someone explain with example, as I still find it difficult to intrepret, which train is 100m and which one is 250 m long?

Thnx in advance.

We can not deduce whether 100m train or 250m train is faster, we just know that the rate of a faster train, whichever it is, is 20 m/s and and the rate of a slower train is 15 m/s.

So, as you can see we can answer the question even not knowing whether 100m train or 250m train is faster. Refer to ANY solution above (they are basically all the same) to see how it can be done.

Re: Two trains of length 100m and 250m run on parallel lines [#permalink]

Show Tags

15 Oct 2013, 18:37

Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
_________________

Re: Two trains of length 100m and 250m run on parallel lines [#permalink]

Show Tags

31 Dec 2013, 05:41

manojgmat wrote:

Two trains of length 100m and 250m run on parallel lines. when they run in the same direction it will take 70 sec to cross each other and when they run in opposite direction, they take 10 sec to cross each other. find the speed of faster train.

Re: Two trains of length 100m and 250m run on parallel lines [#permalink]

Show Tags

07 Feb 2015, 09:36

Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
_________________

Re: Two trains of length 100m and 250m run on parallel lines [#permalink]

Show Tags

12 Sep 2016, 18:37

Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
_________________

Re: Two trains of length 100m and 250m run on parallel lines [#permalink]

Show Tags

12 Sep 2016, 19:06

I thought too much, and figured out it be only 150 m to cross the other train in the same direction, if they start at the same time and if the longer train is faster

Happy New Year everyone! Before I get started on this post, and well, restarted on this blog in general, I wanted to mention something. For the past several months...

It’s quickly approaching two years since I last wrote anything on this blog. A lot has happened since then. When I last posted, I had just gotten back from...

Happy 2017! Here is another update, 7 months later. With this pace I might add only one more post before the end of the GSB! However, I promised that...

The words of John O’Donohue ring in my head every time I reflect on the transformative, euphoric, life-changing, demanding, emotional, and great year that 2016 was! The fourth to...