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Train A traveling at 60 m/hr leaves New York for Dallas at 6 [#permalink]
31 Oct 2012, 03:26

5

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Expert's post

00:00

A

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C

D

E

Difficulty:

35% (medium)

Question Stats:

68% (03:21) correct
31% (02:31) wrong based on 261 sessions

Train A traveling at 60 m/hr leaves New York for Dallas at 6 P.M. Train B traveling at 90 m/hr also leaves New York for Dallas at 9 P.M. Train C leaves Dallas for New York at 9 P.M. If all three trains meet at the same time between New York and Dallas, what is the speed of Train C if the distance between Dallas and New York is 1260 miles?

A. 60 m/hr B. 90 m/hr C. 120 m/hr D. 135 m/hr E. 180 m/hr

Re: Train A traveling at 60 m/hr leaves New York for Dallas at 6 [#permalink]
31 Oct 2012, 03:37

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Expert's post

carcass wrote:

Train A traveling at 60 m/hr leaves New York for Dallas at 6 P.M. Train B traveling at 90 m/hr also leaves New York for Dallas at 9 P.M. Train C leaves Dallas for New York at 9 P.M. If all three trains meet at the same time between New York and Dallas, what is the speed of Train C if the distance between Dallas and New York is 1260 miles?

A. 60 m/hr B. 90 m/hr C. 120 m/hr D. 135 m/hr E. 180 m/hr

Relative speed of train A and train B is 90-60=30 miles per hour, thus B will gain 30 miles every hour compared to A.

Now, in 3 hours (from 6 P.M. to 9 P.M.) that A traveled alone, it covered 60*3=180 miles. To catch up A (to meet A), B will need 180/30=6 hours.

Next, in 6 hours B will cover 6*90=540 miles to the meeting point, thus C covered 1260-540=720 miles.

Since C also needed 6 hours to meet A and B (C also left at 9 P.M), then its rate is 720/6=120 miles per hour.

Re: Train A traveling at 60 m/hr leaves New York for Dallas at 6 [#permalink]
31 Oct 2012, 18:12

5

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Let the time be t for the train (B) traveling at 90 miles/hr So the speed of train (A) traveling at 60 miles/hr will be t+3 (as it started 3 hours before train B)

So using the formula r x t = d Train B --------- 90 x t = 90t (1) Train A --------- 60 x (t+3) = 60t + 180 (2) They are traveling in the same direction. 90t = 60t + 180 30t = 180 t = 6

In 6 hours train A and train B will travel a distance of 540 miles. Train C is running in opposite direction. Total distance is 1260 miles so out of the total distance 540 miles is covered by the two trains so train C has to cover 1260 - 540 miles = 720 miles In order to cover 720 miles in 6 hours train C has to run at the speed of 720/6 = 120 miles/hr

Re: Train A traveling at 60 m/hr leaves New York for Dallas at 6 [#permalink]
03 Dec 2012, 00:48

Train A and Train B meets somewhere between NY and Dallas. This means the miles covered by Train A and Train B were equal during that point. Also, we know that Train A travelled for 3 hours before Train B started.

Distance of Train A + Distance of Train A before Train B starts = Distance of Train B

60t + 60(3) = 90t t = 6 hrs

Train B has covered 90 x 6 = 540 miles when it was met by Train C (which covered the rest of the tracks between Dallas and NY) 1260 - 540 = 720 miles

Train C also started the same time as B. R = 720/6 = 120 mph

Re: Train A traveling at 60 m/hr leaves New York for Dallas at 6 [#permalink]
12 Aug 2013, 05:19

carcass wrote:

Train A traveling at 60 m/hr leaves New York for Dallas at 6 P.M. Train B traveling at 90 m/hr also leaves New York for Dallas at 9 P.M. Train C leaves Dallas for New York at 9 P.M. If all three trains meet at the same time between New York and Dallas, what is the speed of Train C if the distance between Dallas and New York is 1260 miles?

A. 60 m/hr B. 90 m/hr C. 120 m/hr D. 135 m/hr E. 180 m/hr

.. (A) s=60t (B) s = 90(t-3) (C) 1260-s=V(t-3) from these t=9, s=540 and velocity of C = 120
_________________

Re: Train A traveling at 60 m/hr leaves New York for Dallas at 6 [#permalink]
13 Aug 2013, 11:36

Train A traveling at 60 m/hr leaves New York for Dallas at 6 P.M. Train B traveling at 90 m/hr also leaves New York for Dallas at 9 P.M. Train C leaves Dallas for New York at 9 P.M. If all three trains meet at the same time between New York and Dallas, what is the speed of Train C if the distance between Dallas and New York is 1260 miles?

All three trains meet at the same time. If we can find how many miles it takes for the faster train B to catch up to the slower train A, we can figure out at what time all three trains meet.

Train B gains 30miles per hour on train B every hour it travels (90mi/hr - 60 mi/hr = 30 mi/hr) Therefore, if B started when A was 180 mi ahead, it will need 180/30 = 6 hours to catch up to A.

In the 6 hours it takes for train B to catch up to A, it travels 6+90 = 540 miles. Therefore, when A,B and C meet C has traveled 1260-540 = 720 miles. If train C left at 9PM and it took 6 hours for A to catch up to B (and meet C) then the speed of C is d/t ===> 720/6 = 120 mi/hour.

Re: Train A traveling at 60 m/hr leaves New York for Dallas at 6 [#permalink]
31 Dec 2013, 13:17

carcass wrote:

Train A traveling at 60 m/hr leaves New York for Dallas at 6 P.M. Train B traveling at 90 m/hr also leaves New York for Dallas at 9 P.M. Train C leaves Dallas for New York at 9 P.M. If all three trains meet at the same time between New York and Dallas, what is the speed of Train C if the distance between Dallas and New York is 1260 miles?

A. 60 m/hr B. 90 m/hr C. 120 m/hr D. 135 m/hr E. 180 m/hr

Got it

First since A starts 3 hours before it travels 180 miles in that time span

Now, B needs to catch up first so it will need (30)(t) = 180 ---> 6 hours to do so

In 6 hours, B will travel 540 miles

Now, C needs to meet them at the same time

So in 6 hours it will need to cover 1260 - 540 = 720 miles

720 / 6= 120 miles/hour

Hence C is the answer

Provide kudos if you think that this post was helpful

Re: Train A traveling at 60 m/hr leaves New York for Dallas at 6 [#permalink]
07 Jan 2014, 18:38

I understand how the problem is solved but my doubt is: why do they meet at that determinate point with C (720 miles). Why not later? there is no equation that relates the first two trains with the C.

Re: Train A traveling at 60 m/hr leaves New York for Dallas at 6 [#permalink]
07 Jan 2014, 19:13

1

This post received KUDOS

Expert's post

guimooow wrote:

I understand how the problem is solved but my doubt is: why do they meet at that determinate point with C (720 miles). Why not later? there is no equation that relates the first two trains with the C.

Thank you in advance,

It is given to you in the question, we are not assuming it: "If all three trains meet at the same time between New York and Dallas," This is important data that the question stem gives us. It tells us that their speeds are related in a way that all three trains meet together - just when B crosses A, C also passes A and B.
_________________

Re: Train A traveling at 60 m/hr leaves New York for Dallas at 6 [#permalink]
04 Feb 2014, 16:42

A a question for all,

Do RTD PS questions such as this often take you well over 2 minutes? I find it often takes me a minute to read the question - then re-read to write down all the pertinent information. I often need another 2 minutes to complete all of the calculations. Just wondering if it's just me, or if others are in the same boat.

Re: Train A traveling at 60 m/hr leaves New York for Dallas at 6 [#permalink]
04 Feb 2014, 17:13

Well I can only speak for myself but short answer is no, it doesn't at least its a very complex RTD question such as the one with roses for example. But like everything its just practice and its good that you read carefully cause thats the most important thing in RTD.

Cheers J

Posted from my mobile device

gmatclubot

Re: Train A traveling at 60 m/hr leaves New York for Dallas at 6
[#permalink]
04 Feb 2014, 17:13