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bmwhype2
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The car went the first third of the distance at 80 mph, 20 16 Nov 2007, 09:58
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The car went the first third of the distance at 80 mph, 20 third at 24 mph, and the last third at 48 mph. What was the avg speed of the car for the entire trip?



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The car went the first third of the distance at 80 mph, 20 16 Nov 2007, 10:05
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Pick a number here. I would suggest 240.

Assume that the full distance is 720 miles. Therefore, the third of the distance will be 240 miles.

Therefore ,we have

Total time time taken = (240/80) + (240/24) + (240/48) = 3+10+5=18 hrs

18 hrs for 720 miles => average speed of 40mph.
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The car went the first third of the distance at 80 mph, 20 17 Nov 2007, 22:02
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bmwhype2
The car went the first third of the distance at 80 mph, 20 third at 24 mph, and the last third at 48 mph. What was the avg speed of the car for the entire trip?
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1st 1/3 = x/80
2nd 1/3 = x/24
3rd 1/3 = x/48

speed = 3x/(x/80 + x/24 + x/48)
speed = 40 mph
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The car went the first third of the distance at 80 mph, 20 18 Nov 2007, 01:30
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jbs
Pick a number here. I would suggest 240.

Assume that the full distance is 720 miles. Therefore, the third of the distance will be 240 miles.

Therefore ,we have

Total time time taken = (240/80) + (240/24) + (240/48) = 3+10+5=18 hrs

18 hrs for 720 miles => average speed of 40mph.
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this is the way i did it.

however, the OE from GMATClub is different.

[(d/3)/80] + [(d/3)/24] +[(d/3)/48]
becomes
(D/3)*(3/40)


can someone elaborate?
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The car went the first third of the distance at 80 mph, 20 18 Nov 2007, 06:04
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jbs
Pick a number here. I would suggest 240.

Assume that the full distance is 720 miles. Therefore, the third of the distance will be 240 miles.

Therefore ,we have

Total time time taken = (240/80) + (240/24) + (240/48) = 3+10+5=18 hrs

18 hrs for 720 miles => average speed of 40mph.

this is the way i did it.

however, the OE from GMATClub is different.

[(d/3)/80] + [(d/3)/24] +[(d/3)/48]
becomes
(D/3)*(3/40)


can someone elaborate?
Show more


Basically, you are adding the time taken per third of the distance.

For the first third of the distance, time taken is (d/3) / 80. In a similar fashion , you get an equation for time taken for the second as well as the final third of the journey.

Adding the time taken , you get the overall time which when divided by total distance gives you the average speed.

Here's the explanation with numbers.

(d/3)/80 + (d/3)/24 + (d/3)/48 = total time taken = d/(average speed)

Therefore, d/240 + d/72 + d/144 = d/(average speed)

i.e. (3d + 10d + 5d)/720 = d/(average speed)

i.e. average speed = 720/18 = 40.

Hope this helps.



Archived Topic
Hi there,
This topic has been closed and archived due to inactivity or violation of community quality standards. No more replies are possible here.
Where to now? Join ongoing discussions on thousands of quality questions in our Quantitative Questions Forum
Still interested in this question? Check out the "Best Topics" block above for a better discussion on this exact question, as well as several more related questions.
Thank you for understanding, and happy exploring!