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Trains A and B - MGMAT

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Trains A and B - MGMAT

by pkw209 » Fri Apr 30, 2010 12:53 pm
Trains A and B travel at the same constant rate in opposite directions along the same route between Town G and Town H. If, after traveling for 2 hours, Train A passes Train B, how long does it take Train B to travel the entire distance between Town G and Town H?

(1) Train B started traveling between Town G and Town H 1 hour after Train A started traveling between Town H and Town G.

(2) Train B travels at the rate of 150 miles per hour.
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by rn4gmat » Fri Apr 30, 2010 1:44 pm
IMO the answer to this would be E as from both the statements the required distance cannot be derived till we have the speed associated with train A given.

Hope this helps.
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by pkw209 » Mon May 03, 2010 4:05 pm
Unfortunately, that is not the correct answer.
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by Stuart@KaplanGMAT » Mon May 03, 2010 4:29 pm
pkw209 wrote:Trains A and B travel at the same constant rate in opposite directions along the same route between Town G and Town H. If, after traveling for 2 hours, Train A passes Train B, how long does it take Train B to travel the entire distance between Town G and Town H?

(1) Train B started traveling between Town G and Town H 1 hour after Train A started traveling between Town H and Town G.

(2) Train B travels at the rate of 150 miles per hour.
Here's the key piece of info in the stem (which I missed at first too!):

Trains A and B travel at the same constant rate...

So, we know that rate(a) = rate(b). We also know that they meet 2 hours after A starts.

From (2) we have rate(b) but no info about distance.. insufficient.

From (1) we know that A traveled 1 hour longer than B when they met. Since A has been traveling for 2 hours, B must have been traveling for 1 hour.

Well, A and B travel at the same rate. Since it took a combined 3 hours of work to meet (2 hours by A, 1 hour by B), it would also take either train 3 hours of alone time to make the full journey: 1 is sufficient alone, choose (A).
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by pkw209 » Mon May 03, 2010 4:40 pm
The official answer is A. This problem is from a MGMAT CAT. Explanation is below.

(1) SUFFICIENT: This tells us that B started traveling 1 hour after Train A started traveling. From the question we know that Train A had been traveling for 2 hours when the trains passed each other. Thus, train B, which started 1 hour later, must have been traveling for 2 - 1 = 1 hour when the trains passed each other.

Let's call the point at which the two trains pass each other Point P. Train A travels from Town H to Point P in 2 hours, while Train B travels from Town G to Point P in 1 hour. Adding up these distances and times, we have it that the two trains covered the entire distance between the towns in 3 (i.e. 2 + 1) hours of combined travel time. Since both trains travel at the same rate, it will take 3 hours for either train to cover the entire distance alone. Thus, from Statement (1) we know that it will take Train B 3 hours to travel between Town G and Town H.
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by Stuart@KaplanGMAT » Mon May 03, 2010 5:43 pm
pkw209 wrote:The official answer is A. This problem is from a MGMAT CAT. Explanation is below.

(1) SUFFICIENT: This tells us that B started traveling 1 hour after Train A started traveling. From the question we know that Train A had been traveling for 2 hours when the trains passed each other. Thus, train B, which started 1 hour later, must have been traveling for 2 - 1 = 1 hour when the trains passed each other.

Let's call the point at which the two trains pass each other Point P. Train A travels from Town H to Point P in 2 hours, while Train B travels from Town G to Point P in 1 hour. Adding up these distances and times, we have it that the two trains covered the entire distance between the towns in 3 (i.e. 2 + 1) hours of combined travel time. Since both trains travel at the same rate, it will take 3 hours for either train to cover the entire distance alone. Thus, from Statement (1) we know that it will take Train B 3 hours to travel between Town G and Town H.
That's what I said!

:D
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by ssuarezo » Wed May 05, 2010 10:40 am
Stuart Kovinsky wrote:
pkw209 wrote:The official answer is A. This problem is from a MGMAT CAT. Explanation is below.

(1) SUFFICIENT: This tells us that B started traveling 1 hour after Train A started traveling. From the question we know that Train A had been traveling for 2 hours when the trains passed each other. Thus, train B, which started 1 hour later, must have been traveling for 2 - 1 = 1 hour when the trains passed each other.
That's what I said!

:D
Yes Stuart ! .. exactly your words ... they've copied you ...

My problem I think is reading comprehension ( as strange as it may sound)
See:
a- If they travel in opposite directions, how could one pass another?
b- When they say "along the same route between Town G and Town H", does it mean from G to H? leaves G for H? both going to the same place? in opposite directions?

In your scenario, the explanation is clear, but how can I solve this kind of problems when dealing with reading-comprehension data sufficiency?

Thanks for your patience.
Silvia,
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