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A certain car averages 25 miles per gallon of gasoline when

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A certain car averages 25 miles per gallon of gasoline when [#permalink] New post 13 Nov 2007, 17:25
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61% (03:23) correct 39% (02:05) wrong based on 22 sessions
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 50 miles on the highway?

28
30
33
36
38
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 [#permalink] New post 13 Nov 2007, 17:36
OA is D and it came up as the last question from the GMATPrep software.

I also got 37.5 but after I looked at how I got it, I don't think it's right.

Last edited by mnsinger on 13 Nov 2007, 17:47, edited 1 time in total.
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 [#permalink] New post 13 Nov 2007, 17:46
I think the trick is to change the word gallon to hour to easier visualize the problem.

Then you create equations like

R * T = D

25 * T = 10
40 * T = 50

... R * 33/20 = 60 ... and then I get R to be 36.3
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 [#permalink] New post 13 Nov 2007, 18:26
avg = total miles / total gallons


city
r *t = d
25 *t =10
t=2/5 gallons

highway
r*t=d
40 * t = 50
t=5/4

60/(33/20)=36.36 D
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 [#permalink] New post 13 Nov 2007, 18:30
Ans: D

Average Miles/Galon = Total distance driving/total galon used

Driving 10miles in city uses: 10/25 galons
Driving 50miles in highway uses: 50/40 galons
Total gasoline used: 10/25 + 50/40 = 1.65 galons

Average mile/galon = 60/1.65 = 36.3. Thus, ans. is D
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Re: Average Rate Problem [#permalink] New post 30 Mar 2009, 19:02
Why is using weighted average method incorrect in this problem? That's how I got 37.5, but that's not the correct answer.
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Re: Average Rate Problem [#permalink] New post 18 Aug 2009, 02:01
25.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

Total miles driven: 50 + 10 = 60

Total gallons used driving in the city:
10 miles * (1 gal / 25 gal) = 2/5

Total gallons used driving on the highway:
50 mi * (1 gal / 40 gal) = 5/4

60 / (2/5 + 5/4) = 36....

ANSWER: D. 36

As far as the weighted averages goes... its always my policy to keep things in fractions until the last minute... that always helps me to keep the answers as accurate as possible.
Re: Average Rate Problem   [#permalink] 18 Aug 2009, 02:01
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