x(y+1) even ? provided x and y are prime numbers

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crackitpal: Senior | Next Rank: 100 Posts; Posts: 47; Joined: Tue Aug 14, 2007 2:36 pm

by crackitpal » Wed Sep 17, 2008 11:35 am

If x and y are integers, is the value x(y+1) even ?

1. x and y are prime numbers
2. y > 7

I went with A. Because I thought x and y could be any prime numbers from 2 to n. Apart from number 2 all prime numbers are odd. So x(y+1) will have atleast one even number making it even. Am I wrong ? The solution from GMATPrep is C.

Please let me know, I'm going to give exam on Saturday

. Don't feel quite prepared.

Thanks.

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Source: Beat The GMAT — Data Sufficiency | Wed Sep 17, 2008 11:35 am

Stuart@KaplanGMAT: GMAT Instructor; Posts: 3225; Joined: Tue Jan 08, 2008 2:40 pmFollowed by: 614 members; Location: Toronto; Thanked: 1710 times; GMAT Score:800

Re: x(y+1) even ? provided x and y are prime numbers

by Stuart@KaplanGMAT » Wed Sep 17, 2008 12:03 pm

crackitpal wrote:If x and y are integers, is the value x(y+1) even ?

1. x and y are prime numbers
2. y > 7

I went with A. Because I thought x and y could be any prime numbers from 2 to n. Apart from number 2 all prime numbers are odd. So x(y+1) will have atleast one even number making it even. Am I wrong ? The solution from GMATPrep is C.

Please let me know, I'm going to give exam on Saturday . Don't feel quite prepared.

Thanks.

If you pick x = 3 and y = 2, then x(y+1) = 3(3) = 9 which is odd. Only by adding in y>7 do we know that y must be an odd integer, which means that x(y+1) = x(even) = even.

Stuart Kovinsky | Kaplan GMAT Faculty | Toronto

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crackitpal: Senior | Next Rank: 100 Posts; Posts: 47; Joined: Tue Aug 14, 2007 2:36 pm

by crackitpal » Wed Sep 17, 2008 3:04 pm

Thank you very much Stuart!! I think just when your mind is supposed tobe responding, it fails to do so

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alexdallas: Master | Next Rank: 500 Posts; Posts: 108; Joined: Sun Mar 15, 2009 2:04 pm

by alexdallas » Tue Sep 01, 2009 5:06 pm

maybe its too late,,maybe i just don't get it

i don't get this one..can anyone explain it in more detail pls?

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750Goal: Junior | Next Rank: 30 Posts; Posts: 21; Joined: Tue Feb 05, 2008 8:08 pm

by 750Goal » Mon Feb 08, 2010 12:26 pm

Hi Stuart.

In regard to solution provided to this problem long time ago, I may be asking a silly question, but would like
to clarify your solution.

Below you say y>7 = even, but y could be 8,9,10.... and by adding 1 , it would make it even or odd,
therefore it is not only even but also odd... I think I may be missing something here,
could you please clarify at earliest.

Thanks in advance.
Parag

Stuart Kovinsky wrote:
crackitpal wrote:If x and y are integers, is the value x(y+1) even ?

1. x and y are prime numbers
2. y > 7

I went with A. Because I thought x and y could be any prime numbers from 2 to n. Apart from number 2 all prime numbers are odd. So x(y+1) will have atleast one even number making it even. Am I wrong ? The solution from GMATPrep is C.

Please let me know, I'm going to give exam on Saturday . Don't feel quite prepared.

Thanks.
If you pick x = 3 and y = 2, then x(y+1) = 3(3) = 9 which is odd. Only by adding in y>7 do we know that y must be an odd integer, which means that x(y+1) = x(even) = even.

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Stuart@KaplanGMAT: GMAT Instructor; Posts: 3225; Joined: Tue Jan 08, 2008 2:40 pmFollowed by: 614 members; Location: Toronto; Thanked: 1710 times; GMAT Score:800

by Stuart@KaplanGMAT » Mon Feb 08, 2010 12:46 pm

750Goal wrote:Hi Stuart.

In regard to solution provided to this problem long time ago, I may be asking a silly question, but would like
to clarify your solution.

Below you say y>7 = even, but y could be 8,9,10.... and by adding 1 , it would make it even or odd,
therefore it is not only even but also odd... I think I may be missing something here,
could you please clarify at earliest.

Thanks in advance.
Parag

Hi,

that's in reference to why C (together) is the correct answer; only by adding y>7 to the first statement (y is prime) do we know that y must be odd.

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750Goal: Junior | Next Rank: 30 Posts; Posts: 21; Joined: Tue Feb 05, 2008 8:08 pm

by 750Goal » Mon Feb 08, 2010 5:25 pm

Thanks Stuart. Another oversight.

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jeffedwards: Master | Next Rank: 500 Posts; Posts: 109; Joined: Mon Oct 12, 2009 4:58 amFollowed by: 1 member; Thanked: 12 times; GMAT Score:720

by jeffedwards » Mon Feb 08, 2010 6:52 pm

Another way you can think of it is, the statement is asking is at least one of the following statements true: x is even, or y is odd

We know that an even number times any number is even. Since an odd plus one would be even, if either of these statements is true we know that the answer is even.

I like to break up the question before even attacking the problem. This helps me a lot.

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capthan: Newbie | Next Rank: 10 Posts; Posts: 6; Joined: Tue Jun 01, 2010 4:13 am

by capthan » Tue Jun 01, 2010 4:17 am

Hi Stuart,
x can be 2 or 3 and in both case x is prime, and if y>7 it will give you two different answers. IMO the answer is e.

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nikhilkatira: Master | Next Rank: 500 Posts; Posts: 318; Joined: Mon Jul 13, 2009 3:55 am; Thanked: 12 times

by nikhilkatira » Tue Jun 01, 2010 5:56 am

capthan wrote:Hi Stuart,
x can be 2 or 3 and in both case x is prime, and if y>7 it will give you two different answers. IMO the answer is e.

Combining both statements say y is a prime no greater than 7
Therefore y is odd

y+1 = even
X * ( y+1) = even irrespective of x

Best,
Nikhil H. Katira

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preet107guru: Newbie | Next Rank: 10 Posts; Posts: 2; Joined: Sat Dec 18, 2010 8:46 pm

by preet107guru » Sun Dec 19, 2010 8:04 am

Stuart, I still have a doubt. If x can be odd or even, and y is odd, then there are two probable answers and thus the answer must be (E).

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preet107guru: Newbie | Next Rank: 10 Posts; Posts: 2; Joined: Sat Dec 18, 2010 8:46 pm

by preet107guru » Sun Dec 19, 2010 8:07 am

Got it correct. My mistake. The answer is C.

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