What is the greatest common divisor of positive integers a : DS Archive
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# What is the greatest common divisor of positive integers a

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What is the greatest common divisor of positive integers a [#permalink]

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17 Jun 2007, 00:05
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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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17 Jun 2007, 05:05
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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17 Jun 2007, 08:24
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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17 Jun 2007, 08:40
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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17 Jun 2007, 12:30
good catch vshaunak !!

shall remember the '1' forever
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17 Jun 2007, 12:51
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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17 Jun 2007, 21:17
1
KUDOS
I cought you
I cought you
I cought you again
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17 Jun 2007, 21:23
I go with D too.

Caas, whats the OA? Pls give OE if its not D
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17 Jun 2007, 22:37
Unwarranted assumptions guys
Never assume that a and b are different integers if the stem doesn't says so
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17 Jun 2007, 22:49
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, 07:39, edited 2 times in total.
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17 Jun 2007, 23:04
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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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18 Jun 2007, 00:56
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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18 Jun 2007, 01:05
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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18 Jun 2007, 08:07
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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18 Jun 2007, 15:26
Caas,

Can you share your 'My Wrong DS' notes with us? And any other such notes
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06 Mar 2008, 10:30
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
3) same number
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Re: DS - GCD of a and b [#permalink]

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06 Mar 2008, 13:40
I'm still confused. Can someone help me out?

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]

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06 Mar 2008, 13:50
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giantSwan wrote:
I'm still confused. Can someone help me out?

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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Re: DS - GCD of a and b [#permalink]

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06 Mar 2008, 14:03
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]

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06 Mar 2008, 16:53
bmwhype2 wrote:
giantSwan wrote:
I'm still confused. Can someone help me out?

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...
Re: DS - GCD of a and b   [#permalink] 06 Mar 2008, 16:53
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