What is the length of Train A if it crosses Bridge X in 30 seconds

Question

What is the length of Train A if it crosses Bridge X in 30 seconds from the moment it enters the bridge to the moment its final carriage exits the bridge?

(1) Train B, which is 200 meters long, traveling at a constant speed of 20 meters per second, crosses Bridge X in 40 seconds.

(2) The end of Train A, which is traveling at a constant speed, passes a pole in 10 seconds from the moment the train meets the pole.

chetan2u · Accepted Answer

What is the length of Train A if it crosses Bridge X in 30 seconds from the moment it enters the bridge to the moment its final carriage exits the bridge?

So, the train takes 30 seconds to travel a distance that is equal to sum of length of bridge and the train itself. Nothing else known.

(1) Train B, which is 200 meters long, traveling at a constant speed of 20 meters per second, crosses Bridge X in 40 seconds.
This would give us the length of the bridge.
In 40 seconds, the train will travel 20*40 or 800 m, which is length of bridge + legth o ftrain B = > 200+Bridge = 800 or bridge = 600m
Nothing about A.

(2) The end of Train A, which is traveling at a constant speed, passes a pole in 10 seconds from the moment the train meets the pole.
So, the train at constant speed, say C, can travel its length in 10 seconds.
Nothing about bridge or speed of train on bridge.

Combined
We know bridge is 600m and the train takes 30 seconds but we cannot comment till we know the speed on teh bridge.
But, say, the speed was same as given in statement 2, then in 30 seconds, the train would cover thrice its length. 3*L = L+Bridge or 2L = 600 or L=300
However, the speed is not given nor are we told that the train moves throughout with constant speed.
Insufficient

SKDEV · Answer

Yes, I fell for the wrong answer too,
and realized - we cant use information from the statement (2) because in the question we are not given train is crossing the bridge at a constant speed or if the pole is at the bridge, so we cant really use that info

But if both the things are mentioned and we assume train is travelling at constant speed -
using (1) we can arrive at length of bridge = 600m

using (2)
let speed of train A be 'x' and length be 'd' 
then 10 = d/x => d = 10x

now using question stem - 
30 = (length of bridge + length of train) / speed of train
putting values, 30x = 600+d
and using (2) d = 10x, we will arrive at d = 300m

But as mentioned, we cant use statement (2) data at all, answer would be - combined insufficient

abdatta · Answer

This doesn't feel like a good question to me. If the only difference between choosing C and E is whether it's the same velocity used in option 2 and the question, then that's a bit unfair, and doesn't seem to test your data sufficiency skills, but rather tests your reading skills. I hope such questions do not come in the GMAT anymore and that's why this question was retired.

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Posted from my mobile device

playthegame · Answer

Thanks could you please clarify where this was retired, it sounds like this problem is in the new OG guides.

abdatta · Answer

By retired I meant that it's retired from the GMAT and not from the OG. I inferred that it must be retired based on the understanding that all OG questions are retired GMAT questions.

Gemmie · Answer

A: Length of train A
X: Length of bridge X
 S_A : speed of train A

A+X = 30 *  S_A

=> We need to know X and S_A

Statement (1):

Train B's length (200 meters) and speed (20 meters/second).
Train B crosses the bridge in 40 seconds (total distance = speed x time).
This tells us the total distance Train B needs to cover, which includes its own length (200 meters) and the length of Bridge X (X).
We can set up an equation: 200 + X = 20 * 40 (meters).
=> X = 600

But we do not know about  S_A

=> Insufficient

Statement (2):

We know the time it takes for the end of Train A to pass a pole (10 seconds).
This tells us the speed of Train A (length of train passing the pole / time)  if  we knew the length of the train A, which is what we need to find

=> This statement gives no useful info

=> Insufficient

Combining Statements (1) and (2):

Even with both statements, we still lack crucial information.
Statement (1) provides X.
Statement (2) gives no useful info.

=> Insufficient

Therefore, BOTH statements TOGETHER are not sufficient to find the length of Train A.

So the answer is:  Statements (1) and (2) TOGETHER are not sufficient.

Rebaz · Answer

Bunuel   KarishmaB

Can you please explain what  The end of Train A, which is traveling at a constant speed, passes a pole in 10 seconds from the moment the train meets the pole.  means in normal language?

I have no idea what this statement means with the end of Train A passes a pole? What is a pole in this context?

Thanks in advance!

Bunuel · Answer

When the front of Train A reaches a pole, it takes 10 seconds for the entire train to pass the pole. The pole is just a stationary object.

yazlal25 · Answer

According to me the answer should be C.
 
Statement 1 gives us the total distance covered by train B = 20*40 = 800m
Hence length of bridge is 800-200 = 600m

Statement 2 tells us the end of train A travelling at a constant speed passes a pole in 10 sec. So speed of train A = its own lenght (L) / 10

Now combining both we can equate speed of train A
L/10 = (600+L)/30 which gives L = 300m

Can anyone explain why E is the answer?

yazlal25 · Answer

Am I assuming train A has the same speed as train B? No i dont think so!?
I am equating the speed at which train A passes the pole to the speed at which train A passes the bridge. 
So you can say I am assuming train A travels at the same constant speed in both scenarios.

5monthGMAT700a · Answer

Information (1) Analysis:

Train B length = 200m
Train B speed = 20m/s
Time taken for Train B to cross Bridge X = 40 seconds
Using this information, we can find the length of Bridge X.
40 seconds = (200m + Bridge X Length) / 20m/s
800m = 200m + Bridge X Length
Bridge X Length = 600m

Information (2) Analysis:

Train A travels at a constant speed.
The end of Train A passes a pole 10 seconds after the train meets the pole.
When a train passes a pole, it means the train has traveled its own length.
Therefore, Train A Length = Train A Speed * 10 seconds

Goal of the Problem:
To find the length of Train A. We are given that Train A crosses Bridge X in 30 seconds.
30 seconds = (Train A Length + Bridge X Length) / Train A Speed

Now, substitute the length of Bridge X (600m) from Information (1) into this equation:
30 = (Train A Length + 600) / Train A Speed

Also, use the relationship from Information (2) (Train A Length = Train A Speed * 10 seconds) to express Train A Speed:
Train A Speed = (Train A Length) / 10

Now, substitute the expression for Train A Speed into the equation:
30 = (Train A Length + 600) / (Train A Length / 10)
30 = (Train A Length + 600) * 10 / Train A Length
30 * Train A Length = (Train A Length + 600) * 10
3 * Train A Length = Train A Length + 600
2 * Train A Length = 600
Train A Length = 300m

Therefore, the length of Train A is 300m.

IMO, answer should be (C)

LSATGMATKiller · Answer

Basically, the question comes down to if the "constant speed for Train A" in Stmt 2 is the same constant speed Train A is traveling when it crosses the bridge. Although very unclear and not a great question for that reason, just like in verbal reasoning, we can never assume anything (in this case that Train A is traveling at same speed as the speed traveling in Stmt 2) and therefore according to our own rules on assumptions, E is technically correct.

rajatyadav1994 · Answer

I chose C. Basis answers written here, I understand 'mistake' stems from assuming that the speed of train A can be calculated from the statement 2 equation, when statements 1 and 2 are taken together.

We need to make that distinction.

However, I do not understand why we should use Statement 1 to find the length of the bridge, since the statement also indicates that Train B travels at a constant speed. Shouldn't we be arriving at Choice E because there is no usable information in either of the statements, only because constant speed cannot be assumed to be the speed when respective trains cross the bridge?

KarishmaB · Answer

Yes. you are right. The answer here has to be (C).

Statement 2 clearly tells us that t he end of Train A is traveling at a constant speed which means the train is traveling at a constant speed. So it crosses the pole in 10 sec (its own length) and bridge in 30 secs (its own length + length of bridge).

Statement 1 tells us that length of bridge is 600 m.

Time taken to cross 'Own length' : 'Own length + 600' = 1 : 3 hence that is the ratio of 'Own length' : 'Own length + 600' too. 
So we know that length of train A must be 300 m. 
 
Answer (C)

Aashimabhatia · Answer

If this kind of question comes in GMAT, Should I select C or E?
I am really confused.

Aashimabhatia · Answer

Is the answer C or E?

MartyMurray · Answer

Supposedly, the answer is (E) because it's never directly stated. that train A travels at the same speed when crossing the bridge and passing the pole.

However, the use of "is" in statement (2) in " which  is  traveling at a constant speed" indicates that the train "is" traveling at a constant speed, presumably throughout the scenario presented by the question. So, what's logical is that train A is traveling at the same speed when it crosses the bridge and when it passes the pole.

Thus, the most logical answer is (C).

MartyMurray · Answer

It doesn't test your reading skills either.

Carefully reading statement (2), we see that it says that train A " is  traveling at a constant rate." From that, we can logically take that train A "is" traveling at that rate throughout the scenario presented, including when it's on the bridge.

The correct answer is (C).

BinodBhai · Answer

Experts can we assume that train A was travelling at a constant speed on the bridge too and that the pole is on the bridge?  MartyMurray   KarishmaB

What is the length of Train A if it crosses Bridge X in 30 seconds

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What is the length of Train A if it crosses Bridge X in 30 seconds