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What is the greatest common divisor of positive integers a [#permalink]
 17 Jun 2007, 01:05
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What is the greatest common divisor of positive integers a and b?
(1) a and b share exactly one common factor
(2) a and b are both prime numbers
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Director
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Should be 'D'.
Stmt1: Exactly ONE common factor between a and b should be 1.
so GCD =1
SUFF.
Stmt2: Prime numbers have 1 as only common factor. So GCD =1
SUFF.
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Director
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go with B.
st1: the common factor can be any number - so INSUFF
say, 6,10 - common factor is 2
say 3,5 - common factor is 1
st2 : when #s are prime, the only common factor is 1, hence SUFF
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Director
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grad_mba wrote: go with B.
st1: the common factor can be any number - so INSUFF
say, 6,10 - common factor is 2
say 3,5 - common factor is 1
st2 : when #s are prime, the only common factor is 1, hence SUFF
Common factors between 6 and 10 are 1 and 2. Stmt1 says that there is exactly one common factors between a and b.
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Director
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good catch vshaunak !!
shall remember the '1' forever 
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VP
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vshaunak@gmail.com wrote: Should be 'D'.
Stmt1: Exactly ONE common factor between a and b should be 1.
so GCD =1
SUFF.
Stmt2: Prime numbers have 1 as only common factor. So GCD =1
SUFF.
Agree - good answer vshaunak !
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Senior Manager
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I cought you
I cought you
I cought you again 
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Manager
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I go with D too.
Caas, whats the OA? Pls give OE if its not D
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Senior Manager
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Unwarranted assumptions guys
Never assume that a and b are different integers if the stem doesn't says so
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VP
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Caas wrote: Unwarranted assumptions guys Never assume that a and b are different integers if the stem doesn't says so 
nice question caas
Last edited by KillerSquirrel on 18 Jun 2007, 08:39, edited 2 times in total.
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Senior Manager
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vshaunak is absolutely right that stmnt 1 is Suff
but stmnt 2 is not suff since a and b might be equal and in this case there will be two possible common divisors and the integer itself will be the greates coomon divisor.
Ex: a=5, b = 3 GCD = 1
a=7 b=7 GCD is 7
The OA is A
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Director
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Oh my God.
Very good trap. I believe no matter how much I practice, someday somewhere I can fell into trap.....
Thanks CAAS for posting such a nice question. Btw what is the source of the question.
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Senior Manager
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vshaunak@gmail.com wrote: Oh my God.
Very good trap. I believe no matter how much I practice, someday somewhere I can fell into trap.....
Thanks CAAS for posting such a nice question. Btw what is the source of the question.
yeah I made the same mistake
sorry, don't remember the source
it was in my notes "my wrong DS"
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Senior Manager
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Initially thought it was D.. see that it is A.
Very good question. Hopefully, official GMAT does not play such tricks on us 
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Manager
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Caas,
Can you share your 'My Wrong DS' notes with us? And any other such notes
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CEO
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ArvGMAT wrote: Initially thought it was D.. see that it is A. Very good question. Hopefully, official GMAT does not play such tricks on us agreed. i almost fell for it too. except i ran thru my checklist for variable questions 1) -1, 0, 1 2) fraction, radical, integer 3) same number
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You tried your best and you failed miserably. The lesson is 'never try'. -Homer Simpson
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Re: DS - GCD of a and b [#permalink]
06 Mar 2008, 14:40
I'm still confused. Can someone help me out?
I orginally had D.
Your saying for statement 2, if the values are 7 and 7 (two identical primes) it has more then one greatest common divisor? I don't follow. Wouldn't the greatest common divisor just be 7?
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Re: DS - GCD of a and b [#permalink]
06 Mar 2008, 14:50
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giantSwan wrote: I'm still confused. Can someone help me out?
I orginally had D.
Your saying for statement 2, if the values are 7 and 7 (two identical primes) it has more then one greatest common divisor? I don't follow. Wouldn't the greatest common divisor just be 7? scenario A: two different numbers: 2 and 3 yields a GCF of 1 scenario B: same numbers: 2 and 2: GCF 2 we cant determine a hard # for the value of GCF
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You tried your best and you failed miserably. The lesson is 'never try'. -Homer Simpson
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Current Student
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Re: DS - GCD of a and b [#permalink]
06 Mar 2008, 15:03
i will admit..i fell for it too...
i think we should all go to law school before taking GMAT..so that we can prepare ourselves to read first and answer later..
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Re: DS - GCD of a and b [#permalink]
06 Mar 2008, 17:53
bmwhype2 wrote: giantSwan wrote: I'm still confused. Can someone help me out?
I orginally had D.
Your saying for statement 2, if the values are 7 and 7 (two identical primes) it has more then one greatest common divisor? I don't follow. Wouldn't the greatest common divisor just be 7? scenario A: two different numbers: 2 and 3 yields a GCF of 1 scenario B: same numbers: 2 and 2: GCF 2 we cant determine a hard # for the value of GCF oooooooooooh. got it. thanks...
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Re: DS - GCD of a and b
[#permalink]
06 Mar 2008, 17:53
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