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

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

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New post 17 Sep 2010, 04:07
3
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A
B
C
D
E

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  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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Re: GCD [#permalink]

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New post 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.

Answer: E.
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Re: GCD [#permalink]

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New post 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

Hence. (E) is the answer
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Re: GCD [#permalink]

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New post 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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Re: What is the greatest common divisor of a and b? (1) a is [#permalink]

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New post 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

Bunuel please give your inputs on this.
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Re: What is the greatest common divisor of a and b? (1) a is [#permalink]

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New post 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

Bunuel please give your inputs on this.


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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Please read this: Ultimate GMAT Quantitative Megathread | All You Need for Quant | PLEASE READ AND FOLLOW: 12 Rules for Posting!!!

Resources:
GMAT Math Book | Triangles | Polygons | Coordinate Geometry | Factorials | Circles | Number Theory | Remainders; 8. Overlapping Sets | PDF of Math Book; 10. Remainders | GMAT Prep Software Analysis | SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) | Tricky questions from previous years.

Collection of Questions:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


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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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