Author 
Message 
GMAT Instructor
Joined: 04 Jul 2006
Posts: 1253
Location: Madrid

A train always travels at one of two speeds: 160 km/hr in [#permalink]
Show Tags
09 Dec 2007, 07:27
Question Stats:
0% (00:00) correct 0% (00:00) wrong based on 3 sessions
HideShow timer Statistics
A train always travels at one of two speeds: 160 km/hr in rural areas and 40 km/hr in urban areas. Was its average speed from A to B greater than 100 km/hr?
(1) More than 2/3 of the distance from A to B is through rural areas.
(2) The distance from A to B is more than 1000 km. == Message from GMAT Club Team == This is not a quality discussion. It has been retired. If you would like to discuss this question please repost it in the respective forum. Thank you! To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative  Verbal Please note  we may remove posts that do not follow our posting guidelines. Thank you.



CEO
Joined: 17 Nov 2007
Posts: 3525
Concentration: Entrepreneurship, Other
Schools: Chicago (Booth)  Class of 2011

A.
(1) V=(Lu+Lr)/(Lu/40+Lr/160), let k=Lr/Lu=(2/3)/(1/3)=2 ==>
V=(1+2)/(1/40+2/160)=3*160/(4+2)=80 km/h  suff
(2) The distance from A to B does not influence on average speed. insuff



Director
Joined: 09 Jul 2005
Posts: 580

Re: DS: Train [#permalink]
Show Tags
06 Jan 2008, 09:19
The more the train travels through rural a areas, the greater its average speed. Lets calculate the relationship between Lu and Lr so that the average speed is 100 km/h
v=[Lu+Lr]/[Lu/40 + Lr/160]=100 => Lr/Lu=4 which means that in order to reach an average speed of 100 km/h, the train must travel 1/5 of the distance through urban areas and 4/5 of the distance through rural areas. So in order to reach an average speed greater than 100 km/h, the train must travel less than 1/5 of the distance through urban areas and more than 4/5 of the distance through rural areas.
Therefore, we can not conclude anything from S1 nor S2
OA should be E



Director
Joined: 12 Jul 2007
Posts: 855

Re: DS: Train [#permalink]
Show Tags
06 Jan 2008, 09:38
kevincan wrote: A train always travels at one of two speeds: 160 km/hr in rural areas and 40 km/hr in urban areas. Was its average speed from A to B greater than 100 km/hr?
(1) More than 2/3 of the distance from A to B is through rural areas. (2) The distance from A to B is more than 1000 km. 1. Since the distance doesn't matter, let's plug in 480km for the distance and say that exactly 2/3 of the distance from A to B is rural (since we don't know exactly how much it is) 320km = rural 160km = urban 320km/160 = 2 hours in rural 160km/40 = 4 hours in urban 480km/6 hours = 80km/h INSUFFICIENT 2. distance doesn't matter. INSUFFICIENT Answer Ethis is a great example of the test makers favorite trick with averages. It's the TIME spent at each speed that we need to get the average, NOT the distance.



CEO
Joined: 29 Mar 2007
Posts: 2517

Re: DS: Train [#permalink]
Show Tags
06 Jan 2008, 10:04
automan wrote: The more the train travels through rural a areas, the greater its average speed. Lets calculate the relationship between Lu and Lr so that the average speed is 100 km/h
v=[Lu+Lr]/[Lu/40 + Lr/160]=100 => Lr/Lu=4 which means that in order to reach an average speed of 100 km/h, the train must travel 1/5 of the distance through urban areas and 4/5 of the distance through rural areas. So in order to reach an average speed greater than 100 km/h, the train must travel less than 1/5 of the distance through urban areas and more than 4/5 of the distance through rural areas.
Therefore, we can not conclude anything from S1 nor S2
OA should be E I'm not convinced the answer is E. It seems we can conclude something from A. In your analysis you said that in order to have an avg. speed greater than 100km/hr the train needs to travel 4/5 the distance at 160km/hr. Well S1 tells us that it will travel exactly 2/3 of the distance at 160km/hr. So we can answer a definitive NO for s1. The train will not exceed 100km/hr. S2 is irrelevant. Knowing the ratio here is enough. 1. Since the distance doesn't matter, let's plug in 480km for the distance and say that exactly 2/3 of the distance from A to B is rural (since we don't know exactly how much it is) 320km = rural 160km = urban 320km/160 = 2 hours in rural 160km/40 = 4 hours in urban 480km/6 hours = 80km/h You found the answer here... 80km/hr... We know that the train will NEVER exceed 100km/hr b/c the ratio is 2/3. I was w/ A and im still w/ A/



CEO
Joined: 17 Nov 2007
Posts: 3525
Concentration: Entrepreneurship, Other
Schools: Chicago (Booth)  Class of 2011

Re: DS: Train [#permalink]
Show Tags
06 Jan 2008, 10:22
I think Eschn3am is right. (1) More than 2/3 of the distance from A to B is through rural areas. therefore, 80<Vavr<=160. insuff. +1 Kudos
_________________
HOT! GMAT TOOLKIT 2 (iOS) / GMAT TOOLKIT (Android)  The OFFICIAL GMAT CLUB PREP APP, a musthave app especially if you aim at 700+  PrepGame



CEO
Joined: 29 Mar 2007
Posts: 2517

Re: DS: Train [#permalink]
Show Tags
06 Jan 2008, 10:26
walker wrote: I think Eschn3am is right. (1) More than 2/3 of the distance from A to B is through rural areas. therefore, 80<Vavr<=160. insuff. +1 Kudos Ah MAN.... lol I hate it when that happens. def. Kudos to Eschn3am.



Director
Joined: 09 Jul 2005
Posts: 580

Re: DS: Train [#permalink]
Show Tags
06 Jan 2008, 10:36
GMATBLACKBELT wrote: automan wrote: The more the train travels through rural a areas, the greater its average speed. Lets calculate the relationship between Lu and Lr so that the average speed is 100 km/h
v=[Lu+Lr]/[Lu/40 + Lr/160]=100 => Lr/Lu=4 which means that in order to reach an average speed of 100 km/h, the train must travel 1/5 of the distance through urban areas and 4/5 of the distance through rural areas. So in order to reach an average speed greater than 100 km/h, the train must travel less than 1/5 of the distance through urban areas and more than 4/5 of the distance through rural areas.
Therefore, we can not conclude anything from S1 nor S2
OA should be E I'm not convinced the answer is E. It seems we can conclude something from A. In your analysis you said that in order to have an avg. speed greater than 100km/hr the train needs to travel 4/5 the distance at 160km/hr. Well S1 tells us that it will travel exactly 2/3 of the distance at 160km/hr. So we can answer a definitive NO for s1. The train will not exceed 100km/hr. S2 is irrelevant. Knowing the ratio here is enough. 1. Since the distance doesn't matter, let's plug in 480km for the distance and say that exactly 2/3 of the distance from A to B is rural (since we don't know exactly how much it is) 320km = rural 160km = urban 320km/160 = 2 hours in rural 160km/40 = 4 hours in urban 480km/6 hours = 80km/h You found the answer here... 80km/hr... We know that the train will NEVER exceed 100km/hr b/c the ratio is 2/3. I was w/ A and im still w/ A/ You have calculated that if the train travels 2/3 of the distance through rural areas the average speed is 80 km/h . Now imagine that the train travels all the time through rural areas, making S1 true (the train travel 100% of the time through rural areas). In this case the average speed is 160 km/h. Therefore we can no conclude anything. I hope this makes my reasoning more understable.



CEO
Joined: 17 Nov 2007
Posts: 3525
Concentration: Entrepreneurship, Other
Schools: Chicago (Booth)  Class of 2011

Re: DS: Train [#permalink]
Show Tags
06 Jan 2008, 10:38
automan, +1 Kudos
_________________
HOT! GMAT TOOLKIT 2 (iOS) / GMAT TOOLKIT (Android)  The OFFICIAL GMAT CLUB PREP APP, a musthave app especially if you aim at 700+  PrepGame



Director
Joined: 09 Jul 2005
Posts: 580

Re: DS: Train [#permalink]
Show Tags
06 Jan 2008, 10:42
walker wrote: automan, +1 Kudos Another one for you. You always try the most difficult questions!! I have a lot to learn from you



Manager
Joined: 01 Jan 2008
Posts: 221
Schools: Booth, Stern, Haas

Re: DS: Train [#permalink]
Show Tags
06 Jan 2008, 10:46
walker wrote: I think Eschn3am is right. (1) More than 2/3 of the distance from A to B is through rural areas. therefore, 80<Vavr<=160. insuff. +1 Kudos why it is insufficient? I agree 80<Vavr but still you have information to solve problem, am I missing some point?



Director
Joined: 12 Jul 2007
Posts: 855

Re: DS: Train [#permalink]
Show Tags
06 Jan 2008, 10:48
kazakhb wrote: walker wrote: I think Eschn3am is right. (1) More than 2/3 of the distance from A to B is through rural areas. therefore, 80<Vavr<=160. insuff. +1 Kudos why it is insufficient? I agree 80<Vavr but still you have information to solve problem, am I missing some point? We want to know if the average speed is greater than 100km/h. Given the information from statement one we see that the average speed must be greater than 80km/h and less than or equal to 160km/h. 80 < average <= 160 this isn't enough to know if the average is greater than 100 because it could be 81 or 160. and statement 2 doesn't help us at all because the averages will work out to be the same, regardless of distance.



CEO
Joined: 17 Nov 2007
Posts: 3525
Concentration: Entrepreneurship, Other
Schools: Chicago (Booth)  Class of 2011

Re: DS: Train [#permalink]
Show Tags
06 Jan 2008, 10:50
kazakhb wrote: walker wrote: I think Eschn3am is right. (1) More than 2/3 of the distance from A to B is through rural areas. therefore, 80<Vavr<=160. insuff. +1 Kudos why it is insufficient? I agree 80<Vavr but still you have information to solve problem, am I missing some point? Q. Was its average speed from A to B greater than 100 km/hr? 1. 80<Vavr<=160, So, speed greater 100? maybe yes and maybe no
_________________
HOT! GMAT TOOLKIT 2 (iOS) / GMAT TOOLKIT (Android)  The OFFICIAL GMAT CLUB PREP APP, a musthave app especially if you aim at 700+  PrepGame



Senior Manager
Joined: 19 Nov 2007
Posts: 438

Re: DS: Train [#permalink]
Show Tags
06 Jan 2008, 11:04
automan wrote: GMATBLACKBELT wrote: automan wrote: The more the train travels through rural a areas, the greater its average speed. Lets calculate the relationship between Lu and Lr so that the average speed is 100 km/h
v=[Lu+Lr]/[Lu/40 + Lr/160]=100 => Lr/Lu=4 which means that in order to reach an average speed of 100 km/h, the train must travel 1/5 of the distance through urban areas and 4/5 of the distance through rural areas. So in order to reach an average speed greater than 100 km/h, the train must travel less than 1/5 of the distance through urban areas and more than 4/5 of the distance through rural areas.
Therefore, we can not conclude anything from S1 nor S2
OA should be E I'm not convinced the answer is E. It seems we can conclude something from A. In your analysis you said that in order to have an avg. speed greater than 100km/hr the train needs to travel 4/5 the distance at 160km/hr. Well S1 tells us that it will travel exactly 2/3 of the distance at 160km/hr. So we can answer a definitive NO for s1. The train will not exceed 100km/hr. S2 is irrelevant. Knowing the ratio here is enough. 1. Since the distance doesn't matter, let's plug in 480km for the distance and say that exactly 2/3 of the distance from A to B is rural (since we don't know exactly how much it is) 320km = rural 160km = urban 320km/160 = 2 hours in rural 160km/40 = 4 hours in urban 480km/6 hours = 80km/h You found the answer here... 80km/hr... We know that the train will NEVER exceed 100km/hr b/c the ratio is 2/3. I was w/ A and im still w/ A/ You have calculated that if the train travels 2/3 of the distance through rural areas the average speed is 80 km/h . Now imagine that the train travels all the time through rural areas, making S1 true (the train travel 100% of the time through rural areas). In this case the average speed is 160 km/h. Therefore we can no conclude anything. I hope this makes my reasoning more understable. yes that reasoning seems far fetched, but one can't deny it. Whats the OA kevin?
_________________
Underline your question. It takes only a few seconds! Search before you post.



Manager
Joined: 01 Jan 2008
Posts: 221
Schools: Booth, Stern, Haas

Re: DS: Train [#permalink]
Show Tags
27 Nov 2008, 05:01
automan wrote: The more the train travels through rural a areas, the greater its average speed. Lets calculate the relationship between Lu and Lr so that the average speed is 100 km/h
v=[Lu+Lr]/[Lu/40 + Lr/160]=100 => Lr/Lu=4 which means that in order to reach an average speed of 100 km/h, the train must travel 1/5 of the distance through urban areas and 4/5 of the distance through rural areas. So in order to reach an average speed greater than 100 km/h, the train must travel less than 1/5 of the distance through urban areas and more than 4/5 of the distance through rural areas.
Therefore, we can not conclude anything from S1 nor S2
OA should be E can someone describe step by step how it can be simplified? == Message from GMAT Club Team == This is not a quality discussion. It has been retired. If you would like to discuss this question please repost it in the respective forum. Thank you! To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative  Verbal Please note  we may remove posts that do not follow our posting guidelines. Thank you.










