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OG At what speed was a train traveling when it had completed

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by AbeNeedsAnswers » Tue Aug 22, 2017 7:42 pm

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E

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Difficulty—

At what speed was a train traveling on a trip when it had completed half of the total distance of the trip?

(1) The trip was 460 miles long and took 4 hours to complete.
(2) The train traveled at an average rate of 115 miles per hour on the trip.

E
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Source: — Data Sufficiency |

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by Brent@GMATPrepNow » Wed Aug 23, 2017 12:38 pm
AbeNeedsAnswers wrote:At what speed was a train traveling on a trip when it had completed half of the total distance of the trip?

(1) The trip was 460 miles long and took 4 hours to complete.
(2) The train traveled at an average rate of 115 miles per hour on the trip.

E
Target question: At what speed was a train traveling on a trip when it had completed half of the total distance of the trip?

Statement 1: The trip was 460 miles long and took 4 hours to complete.
IMPORTANT: there is nothing in this question to suggest that the train is traveling at a CONSTANT speed. So, there's no way to determine the train's speed when it had competed half of the trip. To see what I mean, consider these two conflicting cases:
Case a: the train travels the first half (230 miles) of the trip at a constant speed 230 mph (for 1 hour), and then travels the second half (230 miles) at a constant speed 230/3 mph (for 3 hours). In this case, the train was traveling at 230 mph when it had completed half of the total distance of the trip
Case b: the train travels at a constant speed of 115 mph for the ENTIRE trip. In this case, the train was traveling at 115 mph when it had completed half of the total distance of the trip
Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT

Statement 2: The train traveled at an average rate of 115 miles per hour on the trip.
NOTE: This statement doesn't provide any new information.
That is, from statement 1, we can already determine that the trains average speed = (460 miles)/(4 hours) = 115 mph.
So, it's POSSIBLE to use the same counter-examples that we used for statement 1:
Case a: the train travels the first half (230 miles) of the trip at a constant speed 230 mph (for 1 hour), and then travels the second half (230 miles) at a constant speed 230/3 mph (for 3 hours). In this case, the train was traveling at 230 mph when it had completed half of the total distance of the trip
Case b: the train travels at a constant speed of 115 mph for the ENTIRE trip. In this case, the train was traveling at 115 mph when it had completed half of the total distance of the trip
Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

Statements 1 and 2 combined
IMPORTANT: Notice that I was able to use the same counter-examples to show that each statement ALONE is not sufficient. So, the same counter-examples will satisfy the two statements COMBINED, which means the combined statements are still NOT SUFFICIENT

Answer: E

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
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by Scott@TargetTestPrep » Fri Aug 25, 2017 9:55 am
AbeNeedsAnswers wrote:At what speed was a train traveling on a trip when it had completed half of the total distance of the trip?

(1) The trip was 460 miles long and took 4 hours to complete.
(2) The train traveled at an average rate of 115 miles per hour on the trip.

E
We need to determine the speed at which a train was traveling when it completed ½ of its trip.

Statement One Alone:

The trip was 460 miles long and took 4 hours to complete.

With the information in statement one, we can determine the average speed of the train for the entire trip.

average speed = total distance/total time

average speed = 460/4

average speed = 115

Although we know that the train traveled at an average speed of 115 mph, we do not know whether the train traveled at a constant rate. Thus, we cannot determine the speed of the train when it completed half of the total distance. Statement one alone is not sufficient to answer the question.

Statement Two Alone:

The train traveled at an average rate of 115 miles per hour on the trip.

We see that statement two provides the same information as statement one and thus is also not sufficient to answer the question.

Statements One and Two Together:

Since statements one and two provide identical information and are each insufficient, we know that the statements together are also insufficient.

Answer: E

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by [email protected] » Thu Aug 31, 2017 10:10 am
Hi AbeNeedsAnswers,

We're asked to find that speed that a train traveling on a trip when it had completed half of the total distance of the trip.

1) The trip was 460 miles long and took 4 hours to complete.

Fact 1 gives us the information to find the AVERAGE speed of the train for the entire trip:
460/4 = 115 miles/hour

However, this is NOT what the question asks for. It's possible that the train changed speeds during the trip, so we don't know whether the train was actually going 115 miles/hour at the half-way point or not.
Fact 1 is INSUFFICIENT

2) The train traveled at an average rate of 115 miles per hour on the trip.

Fact 2 gives us the same information that we determined in Fact 1 - and we still have the same issue as before (we don't know how fast the train was actually going at the half-way point).
Fact 2 is INSUFFICIENT

Combined, we really have just one piece of information: the average speed for the entire trip - and we already know that that is not enough to answer the given question.
Combined, INSUFFICIENT.

Final Answer: E

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