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

It is currently 20 Dec 2014, 04:16

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

problem

  Question banks Downloads My Bookmarks Reviews Important topics  
Author Message
TAGS:
Intern
Intern
avatar
Joined: 17 Dec 2006
Posts: 39
Followers: 0

Kudos [?]: 8 [0], given: 0

problem [#permalink] New post 26 Jun 2008, 07:19
People, I can't do it, please help me

if x positive, is x prime?
1) x3 has exactly four distinct factors.
2) x2-x-6=0

answer is B, is it ok?
Current Student
User avatar
Joined: 12 Jun 2008
Posts: 287
Schools: INSEAD Class of July '10
Followers: 6

Kudos [?]: 37 [0], given: 0

Re: problem [#permalink] New post 26 Jun 2008, 07:39
Answer is (D)

(2) is sufficient since it implies that x=3 and 3 is prime

(1) is sufficient too :

if x is not prime it has at least 2 factors that we can call a and b with a and b different from 1

Then a, b, a^2, a^3, b^2, b^3 are all different factors of x^3 (and there are more) : this is more than 4 factors.

Conclusion : not prime => more than 4 factors and thus 4 factors => prime
Director
Director
avatar
Joined: 23 Sep 2007
Posts: 797
Followers: 5

Kudos [?]: 87 [0], given: 0

Re: problem [#permalink] New post 26 Jun 2008, 07:40
is it x3 and x2 OR x^3 and x^2?
Intern
Intern
avatar
Joined: 17 Dec 2006
Posts: 39
Followers: 0

Kudos [?]: 8 [0], given: 0

Re: problem [#permalink] New post 26 Jun 2008, 07:43
gmatnub, sorry
sure, you are right
Intern
Intern
avatar
Joined: 17 Dec 2006
Posts: 39
Followers: 0

Kudos [?]: 8 [0], given: 0

Re: problem [#permalink] New post 26 Jun 2008, 07:45
thanks a lot, now is correct

if x positive, is x prime?
1) x^3 has exactly four distinct factors.
2) x^2-x-6=0
Intern
Intern
avatar
Joined: 17 Dec 2006
Posts: 39
Followers: 0

Kudos [?]: 8 [0], given: 0

Re: problem [#permalink] New post 26 Jun 2008, 07:49
2) it is clear

1) I think it is also enough, what is wrong here
Manager
Manager
User avatar
Joined: 20 Mar 2008
Posts: 160
Location: USA
Followers: 1

Kudos [?]: 29 [0], given: 0

Re: problem [#permalink] New post 26 Jun 2008, 07:51
It is D.


1) x^3 has only 4 factors --> only possible if x is prime.

2^3 is 8. It has factors 8,4,2,1 Only 4 factors
3^3 is 21. It has factors 27,9,3,1 Only 4 factors

works when x is prime

4^3 is 64. It has factors 64,32,16,8,4,2,1 Greater Than 4 factors
Edit : This explanation is WRONG! Check posts below.

2) x2-x-6=0
(x+2) (x-3)=0
x=3 or x=-2
QS tells us it is +ve x=3 => x is prime.

Last edited by vishy007 on 26 Jun 2008, 08:17, edited 1 time in total.
SVP
SVP
User avatar
Joined: 30 Apr 2008
Posts: 1893
Location: Oklahoma City
Schools: Hard Knocks
Followers: 32

Kudos [?]: 450 [0], given: 32

Re: problem [#permalink] New post 26 Jun 2008, 07:51
This question is from Kaplan GMAT 2005 with CD-ROM and the question as originally posted is not correct.

The correct question:

If x is positive, is x prime?
(1) x^3 has exactly four distinct factors.
(2) x^2 - x - 6 = 0
_________________

------------------------------------
J Allen Morris
**I'm pretty sure I'm right, but then again, I'm just a guy with his head up his a$$.

2 KUDOS received
SVP
SVP
User avatar
Joined: 30 Apr 2008
Posts: 1893
Location: Oklahoma City
Schools: Hard Knocks
Followers: 32

Kudos [?]: 450 [2] , given: 32

Re: problem [#permalink] New post 26 Jun 2008, 07:52
2
This post received
KUDOS
OA is B.

OE:

We have a yes-no question, so we'll need to know if x is always prime or never prime. When working with primes, keep in mind that 2 is the smallest prime number on the only even prime. Other than that, all we know from the stem is that x > 0, so let's evaluate the statements.

(1) Insufficient. Can we find any prime values for x such that x^3 has 4 distinct factors? If x=2, which is prime, then x^3 = 2^3 = 8, which has 4 distict factors (1, ,2, 4, and 8). So it's possible that x is prime. But does it have to be prime? Lets consider other positive numbers with 4 distinct factors. The smalelst such number is 6 (its factors are 1, 2, 3, and 6), so suppose x^3 = 6. In this case, x = \sqrt{6}, which is not even an integer, much less a prime number. So we've found both prime and non-prime values for x that make (1) true. Therefore, (1) is insufficent to determine whether or not x is prime. Eliminate (A) and (D).

(2) Sufficient. In this case, we can solve the equation to find possible values for x. If we subtract 6 from both sides, we get x^2 - x - 6 = 0. Factor the left side using reverse FOIL to get (x-3)(x+2) = 0. So, the equation has one negative root and one positive root, meaning that there are two possible values for x, one negative and one positive. The stem tells us that x must be positive, so there is actually only one possible value of x. Since we could find exactly one value for x, we can answer the question definitely, if we know what x is, we know whether or not it is prime. So Statement (2) is sufficient. Answer B.
_________________

------------------------------------
J Allen Morris
**I'm pretty sure I'm right, but then again, I'm just a guy with his head up his a$$.

Manager
Manager
User avatar
Joined: 20 Mar 2008
Posts: 160
Location: USA
Followers: 1

Kudos [?]: 29 [0], given: 0

Re: problem [#permalink] New post 26 Jun 2008, 08:08
Oops!

I know this trap, yet I keep forgetting time and again.

jallenmorris, thanks for posting OE.
Intern
Intern
avatar
Joined: 17 Dec 2006
Posts: 39
Followers: 0

Kudos [?]: 8 [0], given: 0

Re: problem [#permalink] New post 26 Jun 2008, 08:14
jallenmorris cool!!!
respect,
I didn't even try to do it from that side :twisted:
thanks a lot, my friend
thanks to everybody
Re: problem   [#permalink] 26 Jun 2008, 08:14
    Similar topics Author Replies Last post
Similar
Topics:
1 Experts publish their posts in the topic Problem rite2deepti 2 25 Nov 2010, 00:49
Experts publish their posts in the topic % problem Jinglander 6 25 May 2010, 09:19
Problem.. Sumithra Sen 4 18 Aug 2007, 16:21
Problem rbcola 7 16 Jun 2006, 11:05
Problem rdangol 4 19 Mar 2006, 11:27
Display posts from previous: Sort by

problem

  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®.