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# What is the greatest common divisor of a and b? (1) a is

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Manager
Joined: 04 Jun 2010
Posts: 109
Concentration: General Management, Technology
Schools: Chicago (Booth) - Class of 2013
GMAT 1: 670 Q47 V35
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What is the greatest common divisor of a and b? (1) a is [#permalink]

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17 Sep 2010, 04:07
3
00:00

Difficulty:

55% (hard)

Question Stats:

49% (01:00) correct 51% (00:29) wrong based on 59 sessions

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What is the greatest common divisor of a and b?

(1) a is divisible by 15.

(2) b=2a

Couldn't understand this one. How do you approach this sort of questions?

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Math Expert
Joined: 02 Sep 2009
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17 Sep 2010, 04:11
rafi wrote:
What is the greatest common divisor of a and b?

(1) a is divisible by 15.

(2) b=2a

Couldn't understand this one. How do you approach this sort of questions?

The easiest way would be just to try different numbers. Clearly (1) and (2) alone are not sufficient. Now if we take them together:

If $$a=15$$ then $$b=2a=30$$ and GCD of $$a$$ and $$b$$ is 15 but if $$a=30$$ then $$b=2a=60$$ and GCD of $$a$$ and $$b$$ is 30. Two different answers not sufficient.

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17 Sep 2010, 04:13
rafi wrote:
What is the greatest common divisor of a and b?

(1) a is divisible by 15.

(2) b=2a

Couldn't understand this one. How do you approach this sort of questions?

(1) a is divisible by 15, gives no information about b ... so impossible to conclude the gcd

(2) b=2a only says that the gcd is a. But we dont know what a is.

(1)+(2) ... We know gcd is a ... but again no clue as to what value a has except that it is a multiple of 15. So not enough

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Manager
Joined: 04 Jun 2010
Posts: 109
Concentration: General Management, Technology
Schools: Chicago (Booth) - Class of 2013
GMAT 1: 670 Q47 V35
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17 Sep 2010, 04:16
O.. now I understand. I kept thinking that I know it's a... forgot that it should be only one number
Thanks
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Joined: 03 Apr 2013
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Re: What is the greatest common divisor of a and b? (1) a is [#permalink]

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28 Mar 2017, 06:04
rafi wrote:
What is the greatest common divisor of a and b?

(1) a is divisible by 15.

(2) b=2a

Couldn't understand this one. How do you approach this sort of questions?

I think the question is wrong as far as the wording goes..

The question should say..

What is the VALUE of the common divisor of a and b

otherwise..from statement (2) it can be clearly be said that "a" is the common divisor

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Math Expert
Joined: 02 Sep 2009
Posts: 46201
Re: What is the greatest common divisor of a and b? (1) a is [#permalink]

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28 Mar 2017, 07:33
ShashankDave wrote:
rafi wrote:
What is the greatest common divisor of a and b?

(1) a is divisible by 15.

(2) b=2a

Couldn't understand this one. How do you approach this sort of questions?

I think the question is wrong as far as the wording goes..

The question should say..

What is the VALUE of the common divisor of a and b

otherwise..from statement (2) it can be clearly be said that "a" is the common divisor

Not so.

In data sufficiency problems that ask for the value of a quantity, the data given in the statements are sufficient only when it is possible to determine exactly one numerical value for the quantity.
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Re: What is the greatest common divisor of a and b? (1) a is   [#permalink] 28 Mar 2017, 07:33
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