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Which of the following cannot be the greatest common divisor

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Jason.tuyj
 Post subject: Which of the following cannot be the greatest common divisor
 Post Posted: Tue Jun 07, 2011 11:30 am 
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Which of the following cannot be the greatest common divisor of two postive integers x and y?

a:1
b:x
c:y
d:x-y
e:x+y
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jnelson0612
 Post subject: Re: Which of the following cannot be the greatest common divisor
 Post Posted: Wed Jun 08, 2011 8:12 pm 
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ManhattanGMAT Staff


Posts: 1857
Jason, test some values for x and y and let us know if you can make any eliminations.

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Jamie Nelson
ManhattanGMAT Instructor
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Milanproda
 Post subject: Re: Which of the following cannot be the greatest common divisor
 Post Posted: Tue Jun 14, 2011 11:05 pm 
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I chose random numbers 4 and 8 because I thought that it would be easier to prove the question wrong if I chose simple variables.

(a)- the gcf of of 4 and 8 is 4. the GCF cannot be 1.

(b) if x=4, then the gcf of 4 and 8 is 4. X could be a factor.

(c) if y=4, then the gcf of 4 and 8 is 4. Y could be a factor

(d) x+y=12. 12 cannot be the GCF of the two numbers that are less than 12. This is my answer choice.

(e) x-y can be either -4 or 4, in which case 4 could be the gcf of 4 and 8.

Feel free to destroy my answer choices, I feel like I did not do too well on this one.
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varun_783
 Post subject: Re: Which of the following cannot be the greatest common divisor
 Post Posted: Wed Jun 15, 2011 4:55 am 
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Posts: 263
Hi,

I agree with Milanproda, my answer is also D

One small clarification:

A) 1 - When two numbers do not have any prime factors in common, their GCF is 1. e.g. - 11 and 17 have a GCF of 1

Regards

Sunil
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jnelson0612
 Post subject: Re: Which of the following cannot be the greatest common divisor
 Post Posted: Sun Jun 19, 2011 11:37 pm 
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ManhattanGMAT Staff


Posts: 1857
Just to be clear, the answer is x+y. Jason lists that as E but Sunil and prada list it as D. The number testing method is quite effective with this one.

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Jamie Nelson
ManhattanGMAT Instructor
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