Find all School-related info fast with the new School-Specific MBA Forum

It is currently 22 Oct 2014, 19:25

Close

GMAT Club Daily Prep

Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track
Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.

Events & Promotions

Events & Promotions in June
Open Detailed Calendar

stumped... retired test question is killing me, please help

  Question banks Downloads My Bookmarks Reviews Important topics  
Author Message
TAGS:
Intern
Intern
avatar
Joined: 22 Sep 2007
Posts: 14
Followers: 0

Kudos [?]: 0 [0], given: 0

stumped... retired test question is killing me, please help [#permalink] New post 22 Sep 2007, 23:40
this question is killing me...

if (2/x) + (3/y) = 4 and xy = 5, then 3x + 2y = ?

a. 1/5
b. 1/4
c. 4/5
d. 4
e. 20

-------

Correct answer is supposedly E. I can't for the life of me figure out how to solve this in less than two minutes.

Every time I work it, I end up with a nasty quadratic that takes forever to factor, or solve with the quad formula.

I'm sure it is in my approach... I'm using substitution

if xy = 5, then x = 5/y, then sub into the first equation...

(2/(5/y)) + (3/y) = 4
(2y/5) + (3/y) = 4
2y + (15/y) = 20
2y^2 + 15 = 20y
2y^2 -20y + 15 = 0

This is where it gets really ugly... Quad formula, nope you have to solve something like the sqrt(280), that takes too long to solve in under two minutes.

Factor quickly? What factors of -30 add up to 15?

I must have done something wrong up to this point. I'm just not sure what it is. Thanks!

Any help, step by step, would be VERY much appreciated! Thanks in advance.
Manager
Manager
avatar
Joined: 09 Jul 2007
Posts: 178
Followers: 1

Kudos [?]: 11 [0], given: 0

 [#permalink] New post 22 Sep 2007, 23:48
its very simple prob..

2/x + 3/y= 4
2y+3x/xy=4 (taking lcm)

xy= 5--given

2y+3x/5=4

2y+3x=4*5

2y+3x= 20 i.e. 3x+2y=20
hence it is E
CEO
CEO
User avatar
Joined: 29 Mar 2007
Posts: 2593
Followers: 16

Kudos [?]: 197 [0], given: 0

Re: stumped... retired test question is killing me, please h [#permalink] New post 22 Sep 2007, 23:50
kz wrote:
this question is killing me...

if (2/x) + (3/y) = 4 and xy = 5, then 3x + 2y = ?

a. 1/5
b. 1/4
c. 4/5
d. 4
e. 20

-------

Correct answer is supposedly E. I can't for the life of me figure out how to solve this in less than two minutes.

Every time I work it, I end up with a nasty quadratic that takes forever to factor, or solve with the quad formula.

I'm sure it is in my approach... I'm using substitution

if xy = 5, then x = 5/y, then sub into the first equation...

(2/(5/y)) + (3/y) = 4
(2y/5) + (3/y) = 4
2y + (15/y) = 20
2y^2 + 15 = 20y
2y^2 -20y + 15 = 0

This is where it gets really ugly... Quad formula, nope you have to solve something like the sqrt(280), that takes too long to solve in under two minutes.

Factor quickly? What factors of -30 add up to 15?

I must have done something wrong up to this point. I'm just not sure what it is. Thanks!

Any help, step by step, would be VERY much appreciated! Thanks in advance.


Very common GMAT trick question here. Your used to solving the question by using the ol Two equations two unknowns. This doesn't always work to your advantage on the GMAT.

The question is asking for 3x+2y. So if you can find what 3x+2y = then you know the answer. No need to solve for both X and Y.

here: 2/x+3/y=4 and xy=5. 2/x+3/y=4 (make denom same)

(2y+3x)/xy=4 ---> 2y+3x=4xy ----> xy=5 so---> 2y+3x =4(5)

2y+3x= 20. just change 2y and 3x around and you have:

3x+2y=20.

Ans E.

Remember you don't always have to solve for both X and Y, just 3x + 2y. You must know this concept to do well on the quant section.
Intern
Intern
avatar
Joined: 22 Sep 2007
Posts: 14
Followers: 0

Kudos [?]: 0 [0], given: 0

Re: stumped... retired test question is killing me, please h [#permalink] New post 22 Sep 2007, 23:54
Thank you both!

I won't even tell you how many hours I spent trying to solve this the "hard way". I figured I was missing something VERY simple. Sure enough.

Thanks again...
Intern
Intern
avatar
Joined: 22 Sep 2007
Posts: 14
Followers: 0

Kudos [?]: 0 [0], given: 0

 [#permalink] New post 23 Sep 2007, 00:04
12345678 wrote:
its very simple prob..

2/x + 3/y= 4
2y+3x/xy=4 (taking lcm)



I don't know why... But I stared at that thing forever and couldn't figure out how to get the two fractions to have a common denominator.

Must be the hours quant/verbal studying or something, but I just blanked on the fact that you can multiply fractions independently if you are multiplying by "1"...

(2/x)(y/y) + (3/y)(x/x) = 4, hence
(2y + 3x) / xy = 4
then, as you state it is VERY simple!

Thanks again!
  [#permalink] 23 Sep 2007, 00:04
    Similar topics Author Replies Last post
Similar
Topics:
1 Please help me on this question amirsalehi 2 23 Aug 2014, 10:09
4 Experts publish their posts in the topic This IR Questions Stumped Me - 2 Professors' Statements: NYCAnalyst 11 02 Jun 2012, 13:16
2 Experts publish their posts in the topic Please help me with this question jema08 5 15 Apr 2012, 16:33
DS questions that stumped me from GMATPrep2 ijoshi 0 31 Jul 2014, 00:45
Please help me this question ! cipher 3 06 Jul 2009, 04:36
Display posts from previous: Sort by

stumped... retired test question is killing me, please help

  Question banks Downloads My Bookmarks Reviews Important topics  


GMAT Club MBA Forum Home| About| Privacy Policy| Terms and Conditions| GMAT Club Rules| Contact| Sitemap

Powered by phpBB © phpBB Group and phpBB SEO

Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.