Register    Login    Search    Rss Feeds

Forums Home » Ask An Instructor » GMAT Math » Manhattan GMAT CAT Math


 Page 1 of 1 [ 6 posts ] 



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

Lindsay

Author Message
vietst
 Post subject: Lindsay
 Post Posted: Fri Jan 11, 2008 1:13 pm 
Lindsay can paint 1/x of a certain room in 20 minutes. What fraction of the same room can Joseph paint in 20 minutes if the two of them can paint the room in an hour, working together at their respective rates?

CAT 4 .

1 hour to finish the job, then 20 minutes, finish 1/3
call 1/y the fraction of the same room can J paint in 20 minutes.
1/x + 1/y = 1/3
then 1/y = 1/3 - 1/x = (x-3)/3x
C.
I think It is much shorter.
Top 
StaceyKoprince
 Post subject:
 Post Posted: Mon Jan 14, 2008 6:53 pm 
Offline
ManhattanGMAT Staff


Posts: 6077
Location: San Francisco
Very nice, and yes this is a good technique if you are extremely confident with this type of problem. Many people are more likely to make mistakes with this method, though, than they will when just picking numbers to plug in. But if you are confident you will never make a mistake with this (and remember that the wrong answers will be based upon the common mistakes people make with the algebra), then definitely go ahead and use the algebra!

_________________
Stacey Koprince
Instructor
Director of Online Community
ManhattanGMAT
Top 
as2764
 Post subject: Re: Lindsay
 Post Posted: Fri Mar 04, 2011 1:23 pm 
Offline
Course Students


Posts: 66
vietst wrote:
1 hour to finish the job, then 20 minutes, finish 1/3
call 1/y the fraction of the same room can J paint in 20 minutes.
1/x + 1/y = 1/3
then 1/y = 1/3 - 1/x = (x-3)/3x
C.
I think It is much shorter.

hi -- you've overlooked a minor error.
lindsay and joseph together paint 1 room in 1 hour, so:
1/x + 1/y = 1
1/y = 1-1/x = (x-1)/x --> this is in 1hr=60min
thus, in 20min=60min/3, --> 1/3y = (x-1)/3x <-- right answer

alternatively,
you can say in 20 minutes, L and J together paint a 1/3rd of the room:
1/3*(1/x + 1/y) = 1/3
1/3x + 1/3y = 1/3
1/3y = 1/3 - 1/3x = (x-1)/3x

as Stacey pointed out, unless one is confident, mistakes can happen.

_________________
Ashish
Share not just why the right answer is right, but also why the wrong answers are not.
Top 
tim
 Post subject: Re: Lindsay
 Post Posted: Fri Mar 04, 2011 9:41 pm 
Offline
ManhattanGMAT Staff


Posts: 2242
Location: Southwest Airlines, seat 21C
i think you may be defining your variables differently than the original poster. there are of course several ways to solve most math problems, and you should try as many of them as you can..

_________________
Tim Sanders
Manhattan GMAT Instructor
Top 
as2764
 Post subject: Re: Lindsay
 Post Posted: Sat Mar 05, 2011 4:58 am 
Offline
Course Students


Posts: 66
i think there are two variations of the 'Lindsay' problem; here's the other:
lindsay-paints-t7598.html

the one i posted was for the other variation. sorry for the confusion, but it was really hard to see because the original poster did not include the answer choices w/ the question.

_________________
Ashish
Share not just why the right answer is right, but also why the wrong answers are not.
Top 
jnelson0612
 Post subject: Re: Lindsay
 Post Posted: Sat Mar 05, 2011 7:06 am 
Offline
ManhattanGMAT Staff


Posts: 1857
Thanks as2764.

_________________
Jamie Nelson
ManhattanGMAT Instructor
Top 
Display posts from previous:  Sort by  
 
 Page 1 of 1 [ 6 posts ] 





Who is online

Users browsing this forum: Moz -Google 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: