Last visit was: 25 Apr 2024, 21:24 It is currently 25 Apr 2024, 21:24

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.
Close
Request Expert Reply
Confirm Cancel
SORT BY:
Date
avatar
Intern
Intern
Joined: 16 Nov 2013
Posts: 20
Own Kudos [?]: 84 [15]
Given Kudos: 3
Send PM
Most Helpful Reply
Math Expert
Joined: 02 Sep 2009
Posts: 92915
Own Kudos [?]: 619048 [7]
Given Kudos: 81595
Send PM
User avatar
Manager
Manager
Joined: 22 Oct 2014
Posts: 81
Own Kudos [?]: 153 [5]
Given Kudos: 4
Concentration: General Management, Sustainability
GMAT 1: 770 Q50 V45
GPA: 3.8
WE:General Management (Consulting)
Send PM
General Discussion
User avatar
Manager
Manager
Joined: 13 Jun 2013
Posts: 223
Own Kudos [?]: 1046 [0]
Given Kudos: 14
Send PM
Re: Does positive integer x have a factor f such that 1 < f < x ? [#permalink]
littlewarthog wrote:
The stem can be rephrased as: Is x prime? With this information we can start looking at the statements:

Statement 1: This is obviously not enough, as there is an infinite number of primes greater than 19!. E.G. 19!+1. As consecutive integers are mutually co-prime, 19!+1 must be a prime number.



19!+1 is not a prime no. it is divisible by 71. though, the highlighted portion is correct.
User avatar
Manager
Manager
Joined: 22 Oct 2014
Posts: 81
Own Kudos [?]: 153 [0]
Given Kudos: 4
Concentration: General Management, Sustainability
GMAT 1: 770 Q50 V45
GPA: 3.8
WE:General Management (Consulting)
Send PM
Re: Does positive integer x have a factor f such that 1 < f < x ? [#permalink]
In deed, you are right. Thanks for the heads up. The conclusion stays valid though. For the numbers 2 to 22, it can easily be determined that they are not prime, so statement 2 is still sufficient.
avatar
Intern
Intern
Joined: 16 Nov 2013
Posts: 20
Own Kudos [?]: 84 [0]
Given Kudos: 3
Send PM
Re: Does positive integer x have a factor f such that 1 < f < x ? [#permalink]
littlewarthog wrote:
The stem can be rephrased as: Is x prime? With this information we can start looking at the statements:

Statement 1: This is obviously not enough, as there is an infinite number of primes greater than 19!. E.G. 19!+1. As consecutive integers are mutually co-prime, 19!+1 must be a prime number.

Statement 2: Sufficient. This statement explicitly excluded 19!+1, so we need to check if any value x is co-prime with 19!, in which case x would be prime, since 19! contains all factors<=19.

Because of the same reason, we do not need to actively check all values x <=19! + 19, for which x cannot be prime. One example for this: x=19! + 19:
\(x=19! + 19=18!*19+19=(18!+1)*19\) Therefore, x=19!+19 cannot be prime. The same goes for all other x<=19!+19


Now we need to exclude x=19!+20, x=19!+21 and x=19!+22. Since all of the summands added to 19! are non-prime numbers with factors between 1 and 19, they share factors as 19!. Therefore statement 2 is sufficient to conclude that x cannot be prime.

Answer B.


Why are you saying that we are looking for a prime number? For instance 6 has a factor greater than 1 and less than 6..
User avatar
Manager
Manager
Joined: 22 Oct 2014
Posts: 81
Own Kudos [?]: 153 [1]
Given Kudos: 4
Concentration: General Management, Sustainability
GMAT 1: 770 Q50 V45
GPA: 3.8
WE:General Management (Consulting)
Send PM
Re: Does positive integer x have a factor f such that 1 < f < x ? [#permalink]
1
Bookmarks
Dear gmatmania,

You are right, actually the stem translates to "Is x NOT a prime number?" not "Is x a prime number?" as I have written mistakenly. For your example, as 6 does have factors greater than 1 and smaller than itself, it is not a prime.

However, this doesn't have an impact on the strategy for this question, or even its outcome. As this is a DS question, the question is "Do the statements provide another information to conclude that x is NOT a prime?", which is equivalent to "Do the statements provide enough information to conclude that x is a prime?"

Hope that helps.
avatar
Intern
Intern
Joined: 16 Nov 2013
Posts: 20
Own Kudos [?]: 84 [0]
Given Kudos: 3
Send PM
Re: Does positive integer x have a factor f such that 1 < f < x ? [#permalink]
I can'understand what kind of number are19!+2 and 19!+22 without calculating them.. I can't understand statement 2

Posted from my mobile device
User avatar
Manager
Manager
Joined: 22 Oct 2014
Posts: 81
Own Kudos [?]: 153 [3]
Given Kudos: 4
Concentration: General Management, Sustainability
GMAT 1: 770 Q50 V45
GPA: 3.8
WE:General Management (Consulting)
Send PM
Re: Does positive integer x have a factor f such that 1 < f < x ? [#permalink]
2
Kudos
1
Bookmarks
If all summands of a sum are divisible by a factor, then the sim is also divisible by that factor.

Take for example 19!+6. 19! Is divisible by 6, as 6 is one of its factors and 6 is of course divisible by 6, the sum 19!+6 is also divisible by 6 and therefore no prime number.

With that approach, you can conclude that all numbers from 19!+2 to 19!+22 are not prime numbers, as they all have at least one factor other than 1 and itself.
avatar
Intern
Intern
Joined: 16 Nov 2013
Posts: 20
Own Kudos [?]: 84 [0]
Given Kudos: 3
Send PM
Re: Does positive integer x have a factor f such that 1 < f < x ? [#permalink]
Will i have to consider only integers between 19! +2 and 19!+22? Can i consider 19!+2.1?

Thank you!

Posted from my mobile device
User avatar
Manager
Manager
Joined: 22 Oct 2014
Posts: 81
Own Kudos [?]: 153 [0]
Given Kudos: 4
Concentration: General Management, Sustainability
GMAT 1: 770 Q50 V45
GPA: 3.8
WE:General Management (Consulting)
Send PM
Re: Does positive integer x have a factor f such that 1 < f < x ? [#permalink]
The question stem says that x is a positive integer.

Happy to help. If this is helpful to you, please consider giving me Kudos.
User avatar
Manager
Manager
Joined: 13 Jun 2013
Posts: 223
Own Kudos [?]: 1046 [0]
Given Kudos: 14
Send PM
Re: Does positive integer x have a factor f such that 1 < f < x ? [#permalink]
gmatmania17 wrote:
I can'understand what kind of number are19!+2 and 19!+22 without calculating them.. I can't understand statement 2

Posted from my mobile device


lets consider a simple example lets say we have 5!+2. now 5! = 1*2*3*4*5. thus 5!+2= 1*2*3*4*5 +2 = 2(1*3*4*5 +1). now since the whole expression is a multiple of 2. thus 5!+2 is not a prime number. you can apply the same analogy for 19!+2.

also, for 19!+22. we know that 19! contains both 11 and 2. hence we can easily take out 22 as common factor. thus 19!+22 will not be a prime


gmatmania17 wrote:
Will i have to consider only integers between 19! +2 and 19!+22? Can i consider 19!+2.1?

Thank you!

Posted from my mobile device


NO. since x is an integer. therefore fractional values are not allowed.

i hope it helps.
Magoosh GMAT Instructor
Joined: 28 Dec 2011
Posts: 4452
Own Kudos [?]: 28573 [2]
Given Kudos: 130
Re: Does positive integer x have a factor f such that 1 < f < x ? [#permalink]
2
Kudos
Expert Reply
pepo wrote:
Does positive integer x have a factor f such that 1<f<x?

1) x> 19!

2) 19! + 2<x<19! + 22

Dear pepo,

Please do not start a new thread for a problem already posted. Please search extensively before beginning a brand new thread for a math question, because most GMAT practice math questions in existence have already been posted. You can find this one here:
dose-positive-integer-x-have-a-factor-f-such-that-1-f-x-190858.html
I will ask Bunuel to merge these topics.

Mike :-)
User avatar
Manager
Manager
Joined: 18 Jan 2010
Posts: 210
Own Kudos [?]: 997 [0]
Given Kudos: 9
Send PM
Re: Does positive integer x have a factor f such that 1 < f < x ? [#permalink]
gmatmania17 wrote:
Does positive integer x have a factor f such that 1<f<x ?
(1) x > 19!
(2) 19! +2<=x<=19! +22

Thank you!



Let us take statement 2 First
We had enough explanation for statements such as 19! +2<=x<=19! +22.

x will always have a factor (other than 1 and itself). In other words x is NOT prime.

[say x = 19!+2 --> we can take 2 out. This means that x is divisible by 2. Similarly we can take out 3, 4, 5, .......upto 19. Bottomline: x is composite number]

B is sufficient.

Statement 1

x > 19!

x can be a prime number. In such a case we do not have any factor f.

Or it can be a composite number. In which case there will be some f

Not sufficient.

B is the answer.
Manager
Manager
Joined: 30 Nov 2018
Posts: 64
Own Kudos [?]: 8 [0]
Given Kudos: 194
Send PM
Re: Does positive integer x have a factor f such that 1 < f < x ? [#permalink]
I do not understand the question and the answers mentioned, can someone explain ?
User avatar
Non-Human User
Joined: 09 Sep 2013
Posts: 32681
Own Kudos [?]: 822 [0]
Given Kudos: 0
Send PM
Re: Does positive integer x have a factor f such that 1 < f < x ? [#permalink]
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
GMAT Club Bot
Re: Does positive integer x have a factor f such that 1 < f < x ? [#permalink]
Moderator:
Math Expert
92915 posts

Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne