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# If x and y are prime numbers, which of the following CANNOT

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Manager
Joined: 01 Dec 2011
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If x and y are prime numbers, which of the following CANNOT  [#permalink]

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06 Feb 2012, 04:14
3
5
00:00

Difficulty:

15% (low)

Question Stats:

77% (01:02) correct 23% (00:51) wrong based on 422 sessions

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If x and y are prime numbers, which of the following CANNOT be the sum of x and y?

A) 5
B) 9
C) 13
D) 16
E) 23

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Re: if x and y are prime numbers, which of the following CANNOT  [#permalink]

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06 Feb 2012, 04:21
1
4
fiendex wrote:
if x and y are prime numbers, which of the following CANNOT be the sum of x and y?

A) 5
B) 9
C) 13
D) 16
E) 23

All answer choices but D are odd. In order odd number to be the sum of two prime numbers one must be even, so 2, and another odd.

Now, 23=2+21. Since 21 is not a prime then 23 cannot be written as the sum of two prime numbers.

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Manager
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Re: if x and y are prime numbers, which of the following CANNOT  [#permalink]

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06 Feb 2012, 06:48
2 is the only even prime number --> First 4 choices might be possible
23 may only be possible if 23 - 2 (X or Y) = 21 (X or Y) and 21 is prime --> as 21 is not prime, E is the answer
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Re: if x and y are prime numbers, which of the following CANNOT  [#permalink]

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23 Mar 2012, 10:24
1
hi all,

just thinking aloud,

why is D incorrect? 16 can be written as 14+ 2 ; where 14 is NOT prime number and 2 is a prime number.
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Re: if x and y are prime numbers, which of the following CANNOT  [#permalink]

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23 Mar 2012, 10:27
Hi,

D is incorrect because 16 = 11 + 5 (so it is possible that 16 can be the sum of 2 prime numbers)
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Re: If x and y are prime numbers, which of the following CANNOT  [#permalink]

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09 Sep 2014, 18:28
pappueshwar wrote:
hi all,

just thinking aloud,

why is D incorrect? 16 can be written as 14+ 2 ; where 14 is NOT prime number and 2 is a prime number.

Actually, they are asking about both x & y are prime numbers... 14 is out of question for consideration
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Re: If x and y are prime numbers, which of the following CANNOT  [#permalink]

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25 Nov 2015, 15:36
Hi All,

Since the answer choices are all relatively small, this question can be solved with 'brute force' - we just need to find the 4 answers that COULD be the sum of X and Y - the one that's left will be the correct answer.

We're told that X and Y are PRIME NUMBERS... When taking X+Y, can you get to 4 of the 5 answer choices...

A) 5 --> with 2 and 3
B) 9 --> with 2 and 7
C) 13 --> with 2 and 11
D) 16 --> with 3 and 13

Since we've eliminated those 4 options, there's only one answer left...

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Re: If x and y are prime numbers, which of the following CANNOT  [#permalink]

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04 May 2019, 06:01
Hi all,

I chose the wrong answer A because I was not paying attention.

This is akin to the except questions in Verbal section, that while one finishes calculation and see an answer.

Hope the GMATClub community can learn from my experience.
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Re: If x and y are prime numbers, which of the following CANNOT  [#permalink]

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04 May 2019, 11:57
Hi VictoryVictoria,

Making little mistakes during your studies is not a 'bad' thing - it's a part of the training process (and you can learn from the little mistakes and improve over time). Most silly/little mistakes occur because of a lack of proper note-taking. Doing work "in your head" is the WORST way to approach a GMAT question, so you should never choose to work in that way. By extension, it often helps to summarize the question that is asked - and write that on your pad - so that you have a constant reminder of what you're actually trying to figure out.

When I approached this prompt the first time, the first things I wrote on my pad were:

"X, Y = PRIMES" and "(X+Y) CANNOT be ?"

With just a couple of seconds of note-taking, I have the given information, and the question that is asked, on my pad and I'm ready to get to work.

GMAT assassins aren't born, they're made,
Rich
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Re: If x and y are prime numbers, which of the following CANNOT   [#permalink] 04 May 2019, 11:57
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