Author 
Message 
cesar.rodriguez.blanco

Post subject: DS: Is the integer n odd? Posted: Wed Aug 19, 2009 2:31 pm 


Course Students 

Posts: 144

Is the integer n odd? 1) n is divisible by 3 2) 2n is divisible by twice as many positive integers as n
I do not know why the correct answer is C.





banakida

Post subject: Re: DS: Is the integer n odd? Posted: Thu Aug 20, 2009 1:53 am 


Course Students 

Posts: 4

Are you sure the correct answer is not B?





RonPurewal

Post subject: Re: DS: Is the integer n odd? Posted: Tue Sep 15, 2009 6:54 pm 


ManhattanGMAT Staff 

Posts: 13165

cesar.rodriguez.blanco wrote: Is the integer n odd? 1) n is divisible by 3 2) 2n is divisible by twice as many positive integers as n
I do not know why the correct answer is C. this should be (b). (1) is clearly insufficient. (3: yes; 6: no.) as for (2), that's much more difficult. there's only one way that 2n could be divisible by twice as many positive integers as n itself. here's how that would work: for every factor of n, the multiplication by 2 would create a NEW factor of 2n (i.e., a number that is NOT already a factor of n). this could only happen if NONE of the factors of n contains a '2' in its prime factorization. if that's true, then n is odd. therefore n must be odd. the answer should be (b).
_________________ Pueden hacerle preguntas a Ron en castellano Potete fare domande a Ron in italiano On peut poser des questions Ã£ Ron en franÃ§ais Voit esittÃ¤Ã¤ kysymyksiÃ¤ Ron:lle myÃ¶s suomeksi
Un bon vÃªtement, c'est un passeport pour le bonheur. â€“ Yves SaintLaurent





naren_hn

Post subject: Re: DS: Is the integer n odd? Posted: Tue Dec 28, 2010 10:07 pm 


Course Students 

Posts: 1

Ron,
As you have noted, it is easier to conclude that for a even number, multiplying it by 2 will not result in a new factor and for an odd number, multiplying it by 2 will result in a new factor. But, what does this sentence exactly mean  "is divisible by twice as many positive integers as n"?





mschwrtz

Post subject: Re: DS: Is the integer n odd? Posted: Wed Dec 29, 2010 12:00 pm 


ManhattanGMAT Staff 

Posts: 504

That means that if you list all the factors of n, and all the factors of 2n, the latter list will have twice as many members as will the former.
I can't really add anything to Ron's explanation except an example; I hope that that helps.
Could n be the odd number 35? Sure. 35 has four factors, (1, 5, 7, 35), while 70 has eight, (1, 2, 5, 7, 10, 14, 35, 70). Notice that each of the factors of 70 in bold is twice a corresponding factor of 35. For each of the originalall oddfactors of 35 there is a new corresponding even factor of 70.
But if n were even, then both 1 and 2 would already be among its factors, and doubling 1 would not add a new factor.
Could n be, say, the even number 30? 30 has eight even factors (1, 2, 3, 5, 6, 10, 15, 30), while 60, 2n, has twelve (1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, 60). Twelve ain't twice eight, so n could not be 30.
Notice these pairs of factors of 30: (1, 2), (3, 6), (5, 10), (15, 30). Doubling 30 would not add a new factor corresponding to 1, because 2 is already among the factors. Doubling 30 would not add a new factor corresponding to 3, because 6 is already among the factors, etc.





mba012012

Post subject: Re: DS: Is the integer n odd? Posted: Fri Aug 19, 2011 4:54 pm 


Students 

Posts: 7

you guys are simply superb ! Thanks for a thorough explanation !





RonPurewal

Post subject: Re: DS: Is the integer n odd? Posted: Thu Aug 25, 2011 3:44 am 


ManhattanGMAT Staff 

Posts: 13165

mba012012 wrote: you guys are simply superb ! Thanks for a thorough explanation ! sure
_________________ Pueden hacerle preguntas a Ron en castellano Potete fare domande a Ron in italiano On peut poser des questions Ã£ Ron en franÃ§ais Voit esittÃ¤Ã¤ kysymyksiÃ¤ Ron:lle myÃ¶s suomeksi
Un bon vÃªtement, c'est un passeport pour le bonheur. â€“ Yves SaintLaurent





tsiria

Post subject: Re: DS: Is the integer n odd? Posted: Thu Aug 25, 2011 11:28 am 


Students 

Posts: 5

Another method would be to count factors using primes:
If n is odd: n = 15 = 3 * 5 = 2*2 = 4 factors 2n = 30 = 2*3*5 = 2*2*2 = 8 factors
If n is even: n= 10 = 2*5 = 2*2 = 4 factors 2n=20= (2^2)*5 = 3*2 = 6 factors
Since 2n can only have twice as many factors as n when n is odd, statement 2 is sufficient.





RonPurewal

Post subject: Re: DS: Is the integer n odd? Posted: Sat Sep 03, 2011 8:59 pm 


ManhattanGMAT Staff 

Posts: 13165

tsiria, i'm not seeing how you get from here ... tsiria wrote: Another method would be to count factors using primes:
If n is odd: n = 15 = 3 * 5 = 2*2 = 4 factors 2n = 30 = 2*3*5 = 2*2*2 = 8 factors
If n is even: n= 10 = 2*5 = 2*2 = 4 factors 2n=20= (2^2)*5 = 3*2 = 6 factors ... to here ... [/quote]Since 2n can only have twice as many factors as n when n is odd[/quote] are you implying (and just not saying) that you also tried a considerable number of other examples before coming to this conclusion? because you definitely can't generalize from one example of each type.
_________________ Pueden hacerle preguntas a Ron en castellano Potete fare domande a Ron in italiano On peut poser des questions Ã£ Ron en franÃ§ais Voit esittÃ¤Ã¤ kysymyksiÃ¤ Ron:lle myÃ¶s suomeksi
Un bon vÃªtement, c'est un passeport pour le bonheur. â€“ Yves SaintLaurent





ven2

Post subject: Re: DS: Is the integer n odd? Posted: Tue Jan 03, 2012 3:56 am 


Forum Guests 

Posts: 18

banakida wrote: Are you sure the correct answer is not B? The answer IS b for this one





jnelson0612

Post subject: Re: DS: Is the integer n odd? Posted: Wed Jan 11, 2012 10:24 pm 


ManhattanGMAT Staff 

Posts: 2636

Correct.
_________________ Jamie Nelson ManhattanGMAT Instructor





mail2arifmd

Post subject: Re: DS: Is the integer n odd? Posted: Tue Feb 21, 2012 4:06 am 


Forum Guests 

Posts: 1

Generalisation of tisira explanation Number N represented in factor form = a^p . b^q (where a, b are prime). No.of factors of N = (p+1)(q+1) Note that Odd numbers dont have "2" as a factor to them.
So when we take "2N" then factor form = 2^1 . a^p . b^q No.of factors of 2N = (1+1)(p+1)(q+1) = 2[no.of factors of N] Hence if N is odd then 2N will have twice the number of factors as N.
Consider N as Even. The Factors of which already contains "2" as factor. It could be 2 or 4 or 6 or 8 or any even factor of 2^r N = a^p . b^q . 2^r No.of Factors of N = (p+1)(q+1)(r+1) Now 2N = a^p . b^q . 2^(r+1) No.of Factors of 2N =(p+1)(q+1)(r+1+1) = (p+1)(q+1)(r+2) The relation ship between factors of N and 2N is difficult to show here. It can be double in some cases and might not be double in some other cases.
This explanation is just a prove of concept and not a process for GMAT. The only thing we need to remember for GMAT is:
Rule1) The No.Of factors of 2N = twice the no.of factors of N, when N is Odd.
Rule2) Above rule doesn't fit for Even numbers. It may or may not be twice.





tim

Post subject: Re: DS: Is the integer n odd? Posted: Sun Feb 26, 2012 5:18 am 


ManhattanGMAT Staff 

Posts: 5041 Location: Southwest Airlines, seat 21C

your explanation is correct. i'm not sure this is a rule that one should memorize for the GMAT though, since it is so obscure and unlikely to show up on your test. instead, you should learn how to determine the number of factors of an integer and use that to inform your approach to this problem..
_________________ Tim Sanders Manhattan GMAT Instructor





