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Avg Speed DS

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Avg Speed DS

by srcc25anu » Sat Apr 27, 2013 7:49 am
If during the first two hours of its four-hour trip, a car went at a constant speed of 75 kmh, what was the average speed of the car for the entire trip?
(1) During the last two hours of the trip, the car traveled 10 km further than during the first two hours
(2) The average speed of the car during the last two hours of the trip was 80 kmh

[spoiler]OA:D. Because it assumes total journey is 4 hours long. Can we do that? The question stem only states that avg speed for first 2 hours is X and avg speed for last 2 hours is Y. Dont know if the journey was longer than 4 hours long which is why I went for E[/spoiler][/spoiler]
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by Anju@Gurome » Sat Apr 27, 2013 7:59 am
srcc25anu wrote:If during the first two hours of its four-hour trip, a car went at a constant speed of 75 kmh, what was the average speed of the car for the entire trip?
...
it assumes total journey is 4 hours long. Can we do that?
The problem already said the trip was a four-hour trip.

Hope that helps.
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by Brent@GMATPrepNow » Sat Apr 27, 2013 8:02 am
srcc25anu wrote:If during the first two hours of its four-hour trip, a car went at a constant speed of 75 kmh, what was the average speed of the car for the entire trip?
(1) During the last two hours of the trip, the car traveled 10 km further than during the first two hours
(2) The average speed of the car during the last two hours of the trip was 80 kmh

[spoiler]OA:D. Because it assumes total journey is 4 hours long. Can we do that? The question stem only states that avg speed for first 2 hours is X and avg speed for last 2 hours is Y. Dont know if the journey was longer than 4 hours long which is why I went for E[/spoiler][/spoiler]
Target question: what was the average speed of the car for the entire trip?

Average speed = (total distance traveled)/(total travel time)
We know that total travel time = 4 hours
We also know that, during the first leg of the trip, the car traveled 150 km.
So, to find the total distance traveled, all we need to do is find the distance traveled during the 2nd leg of the trip.

Statement 1: During the last two hours of the trip, the car traveled 10 km further than during the first two hours
If the car traveled 150 km during the first leg, then the car traveled 160 km during the second leg.
So, the car traveled a total of 310 km in 4 hours, which means the average speed was 310/4 kmh
Since we can answer the target question with certainty, statement 1 is SUFFICIENT

Statement 2: The average speed of the car during the last two hours of the trip was 80 kmh
At that rate, the car traveled 160 km during the second leg.
So, the car traveled a total of 310 km in 4 hours, which means the average speed was 310/4 kmh
Since we can answer the target question with certainty, statement 2 is SUFFICIENT

Answer = D

Cheers,
Brent
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by srcc25anu » Sat Apr 27, 2013 8:46 am
Anju@Gurome wrote:
srcc25anu wrote:If during the first two hours of its four-hour trip, a car went at a constant speed of 75 kmh, what was the average speed of the car for the entire trip?
...
it assumes total journey is 4 hours long. Can we do that?
The problem already said the trip was a four-hour trip.

Hope that helps.
I read and re-read the question stem again and again to see what I was missing but could not find this "of FOUR HOUR JOURNEY". :)Now I see it stated big and bold.
Thanks.
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