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

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imane81: Senior | Next Rank: 100 Posts; Posts: 40; Joined: Wed Jan 13, 2010 7:17 pmFollowed by: 1 member; Thanked: 1 times

by imane81 » Tue Feb 16, 2010 4:34 pm

please help on this one

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 other's points of departure.The 2 trains passed each other after traveling for 2 hours, when the two trains passed, which train was nearer to its destination

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Source: Beat The GMAT — Data Sufficiency | Tue Feb 16, 2010 4:34 pm

papgust: Community Manager; Posts: 1537; Joined: Mon Aug 10, 2009 6:10 pmFollowed by: 252 members; Thanked: 653 times

by papgust » Tue Feb 16, 2010 5:30 pm

There is some missing information. Probably the speed of trains.

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sumanr84: Legendary Member; Posts: 758; Joined: Sat Aug 29, 2009 9:32 pmFollowed by: 2 members; Location: Bangalore,India; Thanked: 67 times

by sumanr84 » Tue Feb 16, 2010 10:16 pm

This is called data insufficiency problem

( never mind )

I am on a break !!

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Stuart@KaplanGMAT: GMAT Instructor; Posts: 3225; Joined: Tue Jan 08, 2008 2:40 pmFollowed by: 614 members; Location: Toronto; Thanked: 1710 times; GMAT Score:800

by Stuart@KaplanGMAT » Wed Feb 17, 2010 12:39 am

papgust wrote:There is some missing information. Probably the speed of trains.

It's DS, so I assume that the statements are what's missing!

You're correct though - to solve, what we really need is the relative speed of the trains (we don't need the actual speeds, we just need to know which one is faster - that one will be closer to its destination when they meet).

Stuart Kovinsky | Kaplan GMAT Faculty | Toronto

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dear_xavier: Newbie | Next Rank: 10 Posts; Posts: 6; Joined: Tue Jun 17, 2008 9:14 am; Thanked: 1 times

by dear_xavier » Sat Mar 06, 2010 11:42 am

This is the original question:

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 other'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.

What do you think Stuart?

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hai1: Senior | Next Rank: 100 Posts; Posts: 39; Joined: Tue Oct 23, 2007 9:25 am; Thanked: 1 times; GMAT Score:600

by hai1 » Sat Mar 06, 2010 1:47 pm

1) is not sufficient as relative velocity is needed to determined which travelled more.

2) Not sure what is the speed of other train.

1 & 2 can is enough to solve the problem.

If Q avg speed is 55 mph and both together is 70mph, P should be travelling faster and should be nearr to its destination.

Is the answer C?

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Stuart@KaplanGMAT: GMAT Instructor; Posts: 3225; Joined: Tue Jan 08, 2008 2:40 pmFollowed by: 614 members; Location: Toronto; Thanked: 1710 times; GMAT Score:800

by Stuart@KaplanGMAT » Sat Mar 06, 2010 7:42 pm

dear_xavier wrote:This is the original question:

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 other'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.

What do you think Stuart?

Let's start by analyzing the problem:

They started 250 miles apart, so they collectively traveled 250 miles by the time they passed.

d=250, t=2, therefore their combined rate is 125 mph for those 2 hours.

Next, let's evaluate the statements:

(1) Train P averaged 70mph for the first two hours.

Since Rate P + Rate Q = 125 for those two hours, we can now calculate Rate Q. If we know the 2 rates, we can definitely answer the question: sufficient.

(2) Knowing Q's average speed for the entire trip (i.e. all the way from X to Y) doesn't tell us Q's average speed for the first 2 hours of the trip: insufficient.

(1) is sufficient, (2) isn't: choose A.

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skmarur: Newbie | Next Rank: 10 Posts; Posts: 2; Joined: Tue Feb 09, 2010 10:12 am

by skmarur » Thu Mar 11, 2010 5:05 am

I am slightly confused, the problem does not say that after traveling two hours they have reached their destinations hence the assumption of both trains traveling two hours does not imply the journey is finished. Hence the total time taken to travel 250 kms is not correct
Pls provide a clear answer to this assumption
regards
Sravan

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skmarur: Newbie | Next Rank: 10 Posts; Posts: 2; Joined: Tue Feb 09, 2010 10:12 am

by skmarur » Thu Mar 11, 2010 9:05 am

Sorry
Was not keen in reading so was confused. Your answer is correct
Sravan

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ansumania: Master | Next Rank: 500 Posts; Posts: 301; Joined: Sun Mar 21, 2010 3:18 pm; Thanked: 4 times

by ansumania » Fri May 21, 2010 1:11 am

Stuart Kovinsky wrote:
dear_xavier wrote:This is the original question:

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 other'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.

What do you think Stuart?
Let's start by analyzing the problem:

They started 250 miles apart, so they collectively traveled 250 miles by the time they passed.

d=250, t=2, therefore their combined rate is 125 mph for those 2 hours.

Next, let's evaluate the statements:

(1) Train P averaged 70mph for the first two hours.

Since Rate P + Rate Q = 125 for those two hours, we can now calculate Rate Q. If we know the 2 rates, we can definitely answer the question: sufficient.

(2) Knowing Q's average speed for the entire trip (i.e. all the way from X to Y) doesn't tell us Q's average speed for the first 2 hours of the trip: insufficient.

(1) is sufficient, (2) isn't: choose A.

should not we consider the lengths of the train since we are talking about the two trains passing each other?

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Stuart@KaplanGMAT: GMAT Instructor; Posts: 3225; Joined: Tue Jan 08, 2008 2:40 pmFollowed by: 614 members; Location: Toronto; Thanked: 1710 times; GMAT Score:800

by Stuart@KaplanGMAT » Fri May 21, 2010 10:47 am

ansumania wrote:
should not we consider the lengths of the train since we are talking about the two trains passing each other?

You never have to worry about this on the GMAT - assume that the trains have length 0 (or that you're only concerned with the exact time that the front of the trains meet).

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