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# If x and y are positive integers is y odd?

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If x and y are positive integers is y odd? [#permalink]

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04 Mar 2012, 23:42
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If x and y are positive integers is y odd?

(1) (y+2)!/x! = odd
(2) (y+2)!/x! is greater than 2
[Reveal] Spoiler: OA
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Re: If x and y are positive integers is y odd? [#permalink]

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05 Mar 2012, 00:37
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If x and y are positive integers is y odd?

(1) (y+2)!/x! = odd. Notice that $$\frac{(y+2)!}{x!}=odd$$ can happen only in two cases:

A. $$(y+2)!=x!$$ in this case $$\frac{(y+2)!}{x!}=1=odd$$. For this case $$y$$ can be even: $$y=2=even$$ and $$x=4$$: $$\frac{(y+2)!}{x!}=\frac{24}{24}=1=odd$$;

B. $$y=odd$$ and $$x=y+1$$, in this case $$\frac{(y+2)!}{(y+1)!}=y+2=odd$$. For example, $$y=1=odd$$ and $$x=y+1=2$$: $$\frac{(y+2)!}{x!}=\frac{3!}{2!}=3=odd$$ (basically all the cases like: 3!/2!, 5!/4!, 7!/6!, ... --> y+2=odd, 3, 5, 7, ... --> y=odd, 1, 3, 5, ...).

Not sufficient.

(2) (y+2)!/x! is greater than 2. Clearly insufficient: consider $$y=1=odd$$ and $$x=2$$ OR $$y=2=even$$ and $$x=1$$.

(1)+(2) From (2) we cannot have case A, hence we have case B, which means $$y=odd$$. Sufficient.

Hope it's clear.
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Re: If x and y are positive integers is y odd? [#permalink]

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13 Feb 2014, 04:40
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devinawilliam83 wrote:
If x and y are positive integers is y odd?

(1) (y+2)!/x! = odd
(2) (y+2)!/x! is greater than 2

It's pretty easy to see the answer is (C) if you understand the concept of factorials well.

Note that a! = 1*2*3*4...(a-1)*a
So a! is the product of alternate odd and even numbers.
a!/b! will be an integer only if a >= b

(1) (y+2)!/x! = odd
So y+2 >= x
Two Cases:
Case (i) If y+2 = x, (y+2)!/x! = 1 (odd). y could be odd or even.
Case (ii) If y+2 > x, y+2 must be only one more than x. If y+2 is 2 more than x, you wil have two extra terms in the numerator and one of them will be even. So to ensure that (y+2)!/x! is odd, y+2 must be only 1 more than x and must be odd so all we are left in the numerator is (y+2) which must be odd. y must be odd.
y could be odd or even so this statement alone is not sufficient.

(2) (y+2)!/x! is greater than 2
This means y+2 > x. But we have no idea about whether y is odd or even. Not sufficient.

Using both statements together, we know that only Case (ii) above is possible and hence y must be odd.

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Get started with Veritas Prep GMAT On Demand for $199 Veritas Prep Reviews Manager Joined: 04 Oct 2013 Posts: 180 Concentration: Finance, Leadership GMAT 1: 590 Q40 V30 GMAT 2: 730 Q49 V40 WE: Project Management (Entertainment and Sports) Followers: 6 Kudos [?]: 112 [2] , given: 29 Re: If x and y are positive integers is y odd? [#permalink] ### Show Tags 23 Dec 2013, 04:30 2 This post received KUDOS skamran wrote: also what did you mean by:"(1)+(2) From (2) we cannot have case A, hence we have case B, which means . Sufficient." Will appreciate your help. Let's see if I can help you out. This question involves the knowledge of few concepts. 1) The division of two factorial expressions has to be odd. When does it happen? since a factorial say y! is the product amongst y, (y-1), (y-2) and so on, this sequence will always contain a multiple of two unless y is 1 or 0 (remember 1! =1 and 0! =1). In our case x and y are positive integers thus the lowest value y can assume is 1. If y=1 then (y+2)!=3!. We will not face the zero factorial case thus we can safely get rid of it. Back to our question: if we want to obtain an odd integer from $$\frac{(y+2)!}{x!}$$ either 1. the two numbers have to be equal (entailing 1 as a quotient) EG y=1 ---> (y+2)!=3! x!=3! - result is 1 = odd integer. In this case y is odd y=2 (y+2)!=4! x!=4! - result is 1 = odd integer. In this case y is even. 2. or x! has to be one less than (y+2)! with y as an odd number. EG y=3 (y+2)!=5! x!=4! - 5! upon 4! yields 5 and y must be ODD. Statement one is thus insufficient by itself. 2) this statement doesn't tell us anything interesting besides that x! must be different from (y+2)! y can assume both an even and an odd value. Not sufficient 1+2) Statement 2 conveys us that x! must be different from (y+2)! This said we can get rid of case one in our statement 1 analysis. Now we are sure that Y is an odd integer. Sufficient _________________ learn the rules of the game, then play better than anyone else. Intern Joined: 13 Jan 2013 Posts: 5 Followers: 0 Kudos [?]: 12 [1] , given: 2 Re: If x and y are positive integers is y odd? [#permalink] ### Show Tags 10 Sep 2013, 12:38 1 This post received KUDOS devinawilliam83 wrote: If x and y are positive integers is y odd? (1) (y+2)!/x! = odd (2) (y+2)!/x! is greater than 2 From (1): assume: x = y + 2, so (y + 2)!/x! = 1 = odd y can odd or even, so (1) Insufficient. From (2): assume x = y + 1, so (y + 2)!/x! = y + 2 > 2 because y is a positive integer y can odd or even, so (2) Insufficient. From (1) + (2): (y + 2)!/x! > 2 and (y + 2)!/x! = odd Assume: (y + 2)!/x! = 3, so (y + 2)! =x!*(2k + 1) with k >=1 If x = y + 2, so 1 = 2k + 1 (wrong because k >=1) If x = y + 1, so y + 2 = 2k + 1, therefore y = odd If x = y, so (y + 1)(y + 2) = 2k + 1 (wrong) because the product of 2 consecutive integers is even. If x = y - 1, so y(y + 1)(y + 2) = 2k + 1 (wrong) because the product is always even. Only one solution is right: If x = y + 1, so y + 2 = 2k + 1, therefore y = odd Answer is C. Intern Joined: 12 Jul 2012 Posts: 31 Followers: 0 Kudos [?]: 88 [0], given: 1 Re: If x and y are positive integers is y odd? [#permalink] ### Show Tags 24 Sep 2012, 23:17 very good explanation Math Expert Joined: 02 Sep 2009 Posts: 34862 Followers: 6485 Kudos [?]: 82733 [0], given: 10116 Re: If x and y are positive integers is y odd? [#permalink] ### Show Tags 17 Jul 2013, 00:22 From 100 hardest questions Bumping for review and further discussion. _________________ Intern Joined: 06 Jan 2013 Posts: 2 GMAT 1: Q V0 GPA: 3.34 WE: Analyst (Computer Software) Followers: 0 Kudos [?]: 0 [0], given: 9 Re: If x and y are positive integers is y odd? [#permalink] ### Show Tags 19 Aug 2013, 08:08 Bunuel wrote: If x and y are positive integers is y odd? (1) (y+2)!/x! = odd. Notice that $$\frac{(y+2)!}{x!}=odd$$ can happen only in two cases: A. $$(y+2)!=x!$$ in this case $$\frac{(y+2)!}{x!}=1=odd$$. For this case $$y$$ can be even: $$y=2=even$$ and $$x=4$$: $$\frac{(y+2)!}{x!}=\frac{24}{24}=1=odd$$; B. $$y=odd$$ and $$x=y+1$$, in this case $$\frac{(y+2)!}{(y+1)!}=y+2=odd$$. For example, $$y=1=odd$$ and $$x=y+1=2$$: $$\frac{(y+2)!}{x!}=\frac{3!}{2!}=3=odd$$ (basically all the cases like: 3!/2!, 5!/4!, 7!/6!, ... --> y+2=odd, 3, 5, 7, ... --> y=odd, 1, 3, 5, ...). Not sufficient. (2) (y+2)!/x! is greater than 2. Clearly insufficient: consider $$y=1=odd$$ and $$x=2$$ OR $$y=2=even$$ and $$x=1$$. (1)+(2) From (2) we cannot have case A, hence we have case B, which means $$y=odd$$. Sufficient. Answer: C. Hope it's clear. *********** Hi Bunuel, **B. y=odd and x=y+1, in this case \frac{(y+2)!}{(y+1)!}=y+2=odd. For example, y=1=odd and x=y+1=2: \frac{(y+2)!}{x!}=\frac{3!}{2!}=3=odd (basically all the cases like: 3!/2!, 5!/4!, 7!/6!, ... --> y+2=odd, 3, 5, 7, ... --> y=odd, 1, 3, 5, ...). ** You mentioned y=odd, 1, 3, 5, ...; and (1)+(2) From (2) we cannot have case A, hence we have case B, which means $$y=odd$$. Sufficient. Are you considering y=1 as odd? can you explain how y=1 satisfies the solution. Math Expert Joined: 02 Sep 2009 Posts: 34862 Followers: 6485 Kudos [?]: 82733 [0], given: 10116 Re: If x and y are positive integers is y odd? [#permalink] ### Show Tags 19 Aug 2013, 08:17 abhisheksriv85 wrote: Bunuel wrote: If x and y are positive integers is y odd? (1) (y+2)!/x! = odd. Notice that $$\frac{(y+2)!}{x!}=odd$$ can happen only in two cases: A. $$(y+2)!=x!$$ in this case $$\frac{(y+2)!}{x!}=1=odd$$. For this case $$y$$ can be even: $$y=2=even$$ and $$x=4$$: $$\frac{(y+2)!}{x!}=\frac{24}{24}=1=odd$$; B. $$y=odd$$ and $$x=y+1$$, in this case $$\frac{(y+2)!}{(y+1)!}=y+2=odd$$. For example, $$y=1=odd$$ and $$x=y+1=2$$: $$\frac{(y+2)!}{x!}=\frac{3!}{2!}=3=odd$$ (basically all the cases like: 3!/2!, 5!/4!, 7!/6!, ... --> y+2=odd, 3, 5, 7, ... --> y=odd, 1, 3, 5, ...). Not sufficient. (2) (y+2)!/x! is greater than 2. Clearly insufficient: consider $$y=1=odd$$ and $$x=2$$ OR $$y=2=even$$ and $$x=1$$. (1)+(2) From (2) we cannot have case A, hence we have case B, which means $$y=odd$$. Sufficient. Answer: C. Hope it's clear. *********** Hi Bunuel, **B. y=odd and x=y+1, in this case \frac{(y+2)!}{(y+1)!}=y+2=odd. For example, y=1=odd and x=y+1=2: \frac{(y+2)!}{x!}=\frac{3!}{2!}=3=odd (basically all the cases like: 3!/2!, 5!/4!, 7!/6!, ... --> y+2=odd, 3, 5, 7, ... --> y=odd, 1, 3, 5, ...). ** You mentioned y=odd, 1, 3, 5, ...; and (1)+(2) From (2) we cannot have case A, hence we have case B, which means $$y=odd$$. Sufficient. Are you considering y=1 as odd? can you explain how y=1 satisfies the solution. Well 1 is an odd number. If $$y=1=odd$$ and $$x=y+1=2$$, then $$\frac{(y+2)!}{x!}=\frac{3!}{2!}=3=odd$$. _________________ Intern Joined: 06 Jan 2013 Posts: 2 GMAT 1: Q V0 GPA: 3.34 WE: Analyst (Computer Software) Followers: 0 Kudos [?]: 0 [0], given: 9 Re: If x and y are positive integers is y odd? [#permalink] ### Show Tags 19 Aug 2013, 08:20 Bunuel wrote: abhisheksriv85 wrote: Bunuel wrote: If x and y are positive integers is y odd? (1) (y+2)!/x! = odd. Notice that $$\frac{(y+2)!}{x!}=odd$$ can happen only in two cases: A. $$(y+2)!=x!$$ in this case $$\frac{(y+2)!}{x!}=1=odd$$. For this case $$y$$ can be even: $$y=2=even$$ and $$x=4$$: $$\frac{(y+2)!}{x!}=\frac{24}{24}=1=odd$$; B. $$y=odd$$ and $$x=y+1$$, in this case $$\frac{(y+2)!}{(y+1)!}=y+2=odd$$. For example, $$y=1=odd$$ and $$x=y+1=2$$: $$\frac{(y+2)!}{x!}=\frac{3!}{2!}=3=odd$$ (basically all the cases like: 3!/2!, 5!/4!, 7!/6!, ... --> y+2=odd, 3, 5, 7, ... --> y=odd, 1, 3, 5, ...). Not sufficient. (2) (y+2)!/x! is greater than 2. Clearly insufficient: consider $$y=1=odd$$ and $$x=2$$ OR $$y=2=even$$ and $$x=1$$. (1)+(2) From (2) we cannot have case A, hence we have case B, which means $$y=odd$$. Sufficient. Answer: C. Hope it's clear. *********** Hi Bunuel, **B. y=odd and x=y+1, in this case \frac{(y+2)!}{(y+1)!}=y+2=odd. For example, y=1=odd and x=y+1=2: \frac{(y+2)!}{x!}=\frac{3!}{2!}=3=odd (basically all the cases like: 3!/2!, 5!/4!, 7!/6!, ... --> y+2=odd, 3, 5, 7, ... --> y=odd, 1, 3, 5, ...). ** You mentioned y=odd, 1, 3, 5, ...; and (1)+(2) From (2) we cannot have case A, hence we have case B, which means $$y=odd$$. Sufficient. Are you considering y=1 as odd? can you explain how y=1 satisfies the solution. Well 1 is an odd number. If $$y=1=odd$$ and $$x=y+1=2$$, then $$\frac{(y+2)!}{x!}=\frac{3!}{2!}=3=odd$$. Thanks Bunuel... got it!! Intern Joined: 04 Sep 2013 Posts: 19 Followers: 0 Kudos [?]: 0 [0], given: 5 Re: If x and y are positive integers is y odd? [#permalink] ### Show Tags 16 Sep 2013, 07:17 [quote="Bunuel"]If x and y are positive integers is y odd? I am sorry i don't want to sound dumb, i just want to know how you assumed the cases above?? also what did you mean by:"(1)+(2) From (2) we cannot have case A, hence we have case B, which means . Sufficient." Will appreciate your help. Manager Joined: 25 Oct 2013 Posts: 173 Followers: 1 Kudos [?]: 50 [0], given: 56 Re: If x and y are positive integers is y odd? [#permalink] ### Show Tags 03 Feb 2014, 05:54 (1): $$\frac{(y+2)!}{x!}$$ is odd. Clearly $$x<=(y+2)$$. Let x=y+2 then $$\frac{(y+2)!}{x!}$$ =1 y can be even or odd. so (1) is insufficient (2): $$\frac{(y+2)!}{x!} > 2$$ so x has to be less than y+2 Let x=y+1 then $$\frac{(y+2)!}{x!} > 2$$ is $$y+2>2$$ From this y can be any +ve integer. insufficient. (1)+(2) $$\frac{(y+2)!}{x!} > 2$$ and also odd. Continuing the analysis from above, if y+2>2 and y+2 is odd, then y has to be odd. Next Let x=y then $$\frac{(y+2)!}{x!} > 2$$ is (y+2)(y+1) > 2. Notice (y+2) & (y+1) are consecutive integers. their product will always be even. This contradicts (1)+(2) hence x cannot be equal to y. similarly x cannot be equal to y-1 and so on. x has to be equal to y+1. and y must be odd. We have a concrete answer, y is odd. (1)+(2) is sufficient. _________________ Click on Kudos if you liked the post! Practice makes Perfect. Intern Joined: 18 Dec 2014 Posts: 2 Followers: 0 Kudos [?]: 0 [0], given: 1 Re: If x and y are positive integers is y odd? [#permalink] ### Show Tags 19 Dec 2014, 18:20 i igore the possibility that (y+2)! is equal to x! so i choose A... a little difficult~~ Director Joined: 07 Aug 2011 Posts: 582 Concentration: International Business, Technology GMAT 1: 630 Q49 V27 Followers: 3 Kudos [?]: 350 [0], given: 75 is Y Odd ? [#permalink] ### Show Tags 24 Dec 2014, 20:00 if X and Y are positive integers , is Y odd ? 1) (Y+2)!/X! = Odd 2) (Y+2)!/X! is greater than 2 . _________________ Thanks, Lucky _______________________________________________________ Kindly press the to appreciate my post !! Intern Joined: 05 Dec 2014 Posts: 3 Followers: 0 Kudos [?]: 2 [0], given: 1 is Y Odd ? [#permalink] ### Show Tags 25 Dec 2014, 02:35 Case 1: (Y+2)!/X! = Odd If Y=3, then the above equation can only be odd when X= (Y+2)-1 which is 4. So you get 5!/4! =5. If Y=2, and X=4, 4!/4! = 1 which is also odd. Hence, Insufficient. Case 2:(Y+2)!/X! is greater than 2 This holds for multiple values of Y and X. Hence clearly, Insufficient. Together, if it is greater than 2 then it means Y has to be odd. Hence C! _________________ If my post is helpful/correct, consider giving Kudos Math Expert Joined: 02 Sep 2009 Posts: 34862 Followers: 6485 Kudos [?]: 82733 [0], given: 10116 Re: If x and y are positive integers is y odd? [#permalink] ### Show Tags 25 Dec 2014, 02:51 Lucky2783 wrote: if X and Y are positive integers , is Y odd ? 1) (Y+2)!/X! = Odd 2) (Y+2)!/X! is greater than 2 . Merging topics. Please read carefully and follow: rules-for-posting-please-read-this-before-posting-133935.html Pay attention to rules 1, and 3! Thank you. _________________ Manager Joined: 22 Aug 2014 Posts: 204 Followers: 0 Kudos [?]: 10 [0], given: 49 Re: If x and y are positive integers is y odd? [#permalink] ### Show Tags 09 Mar 2015, 09:55 Hey Bunuel, I solved the problem by plugging in values combining statement 1 and 2, 10/2=5 and 9/3=3 in this case answer will be E. Please explain where I did wrong! Math Expert Joined: 02 Sep 2009 Posts: 34862 Followers: 6485 Kudos [?]: 82733 [0], given: 10116 Re: If x and y are positive integers is y odd? [#permalink] ### Show Tags 09 Mar 2015, 10:04 ssriva2 wrote: Hey Bunuel, I solved the problem by plugging in values combining statement 1 and 2, 10/2=5 and 9/3=3 in this case answer will be E. Please explain where I did wrong! Please elaborate how you got these values. Thank you. _________________ EMPOWERgmat Instructor Status: GMAT Assassin/Co-Founder Affiliations: EMPOWERgmat Joined: 19 Dec 2014 Posts: 7477 Location: United States (CA) GMAT 1: 800 Q51 V49 GRE 1: 340 Q170 V170 Followers: 330 Kudos [?]: 2212 [0], given: 162 Re: If x and y are positive integers is y odd? [#permalink] ### Show Tags 09 Mar 2015, 11:56 Hi ssriva2, You should double-check your math. Neither of those 2 examples 'fits' Fact 1. GMAT assassins aren't born, they're made, Rich _________________ # Rich Cohen Co-Founder & GMAT Assassin # Special Offer: Save$75 + GMAT Club Tests

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Re: If x and y are positive integers is y odd? [#permalink]

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07 Apr 2016, 11:53
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