Register    Login    Search    Rss Feeds

Forums Home » Ask An Instructor » GMAT Math » GMAT Prep Math


 Page 1 of 1 [ 6 posts ] 



  Print view Print view Previous topic Previous topic | Next topic Next topic
 

Please help Gmat prep 2 PS: a certain car averages..

Author Message
yichunette
 Post subject: Please help Gmat prep 2 PS: a certain car averages..
 Post Posted: Wed May 28, 2008 5:04 pm 
A certain car averages 25 miles per gallon of gasolin when driven in the city and 40 miles per gallon when driven on the highway. According to these rate, 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?

Answer: 36

Please explain how to solve this problem.

Thank you!

Yi
Top 
rfernandez
 Post subject:
 Post Posted: Fri May 30, 2008 5:55 am 
Offline
ManhattanGMAT Staff


Posts: 386
The first step is to figure out how much gasoline the car uses on each leg of the trip. We can find this by dividing the number of miles traveled by the miles per gallon.

City: 10 miles / 25 miles per gallon = 2/5 gallon
Highway: 50 miles / 40 miles per gallon = 5/4 gallon

So the car uses 2/5 + 5/4 gallons total, or 33/20 gallons.

The average miles per gallon for the entire trip can be calculated by dividing total miles by total gallons:

60 miles / (33/20) gallons = 400/11 mpg, or roughly 36 mpg.


Rey
Top 
yichunette
 Post subject: Thanks
 Post Posted: Thu Jun 05, 2008 9:38 am 
Thank you very much for your help!
Top 
rfernandez
 Post subject:
 Post Posted: Fri Jun 06, 2008 5:19 am 
Offline
ManhattanGMAT Staff


Posts: 386
You're welcome!
Top 
floubin
 Post subject: Re: Please help Gmat prep 2 PS: a certain car averages..
 Post Posted: Wed Dec 02, 2009 1:32 am 
Offline
Forum Guests


Posts: 1
Why can't you solve this problem this way:

10/60 of the trip in the city, 50/60 of the trip on the highway

(1/6)*25 + (5/6)*40 = 37.5 or ~38

That would make the answer E, not D

?
Top 
StaceyKoprince
 Post subject: Re: Please help Gmat prep 2 PS: a certain car averages..
 Post Posted: Tue Feb 09, 2010 6:41 pm 
Offline
ManhattanGMAT Staff


Posts: 6077
Location: San Francisco
For these kinds of problems, you can't just calculate an average using the two rates given. The overall mpg is going to be (total mileage) / (total gallons), so you actually have to calculate total mileage and total gallons.

_________________
Stacey Koprince
Instructor
Director of Online Community
ManhattanGMAT
Top 
Display posts from previous:  Sort by  
 
 Page 1 of 1 [ 6 posts ] 





Who is online

Users browsing this forum: No registered users and 0 guests

 
 

 
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to: