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Gmat Prep (is this weighted average?)

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Gmat Prep (is this weighted average?)

by aosh » Thu Aug 02, 2007 6:10 am
A certain car averages 25 miles per gallon of gasoline when driven in the city and 40 miles per gallon when driven on the highway. According to these rates, which of the following is closest to the number of miles per gallon that the car averages when it is driven 10 miles in the city and then 50 miles on the highway?

- 28
- 30
- 33
- 36
- 38

Thanks in advance
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by g_beatthegmat » Thu Aug 02, 2007 7:20 am
My approach-

In city, the trip uses 10/25 Gallons (as 25miles consumes 1 Gallon, 10 miles would consume 10/25)
On the highway, the trip uses 50/40 Gallons (as 40miles consumes 1 Gallon, 50 miles would consume 50/40)

So that gives us-
Total gas consumed = 10/25 + 50/40
=> 33/20 Gallons

Total miles covered in the trip = 10 (in the city) + 50 (on the highway)
= > 60 miles

So, to calculate the avg.,
33/20 gallons consumed for 60 miles
1 gallon consumed for => 60/(33/20)
=> 36.3
~ 36
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by California4jx » Sat Jun 27, 2009 8:54 am
it seems like a weighted average problem.

The two averages (balancing points) are 25 and 40.

Drove in the city = 10 miles
Drove in the highway = 50 miles

it drove more miles on highway so the average will be more tilted (weighted) towards 50. But how much ?

The answer to how much can be found with ratios - it drove (50/60) or 5/6 of the range of speed between 25 and 40 ..

therefore. 5/6 (40-25) => 5/6 (15) => 5 * (2.5) = 12.5

the average will be tilted 12.5 more towards 40 miles.

Therefore, 25 + 12.5 = 37.5 - rounding off >> 38

(E) - let me know the OA
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by DanaJ » Sat Jun 27, 2009 9:01 am
aosh - one of the forum rules is that you should try to do a quick search f keywords before posting a question. All questions from GMATprep have been answered on this forum! I found two links in 15 seconds:

https://www.beatthegmat.com/gmat-prep-tr ... 32832.html

https://www.beatthegmat.com/rate-and-ave ... t9005.html
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