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Which of the following cannot be the GCD of two positive integers x and y? a 1 b x c y d x-y e x+y

Divisor of a positive integer cannot be more than that integer (for example integer 4 doesn't have a divisor more than 4, the largest divisor it has is 4 itself), so greatest common divisor of two positive integers x and y can not be more than x or y.

The greatest common divisor of x and y must be a divisor of x, so it can't be larger than x. Since x+y is larger than x, it cannot be the greatest common divisor of x and y.
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Re: Which of the following CANNOT be the greatest common divisor [#permalink]

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24 Jul 2012, 10:42

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Manhattan's way of visualizing the GCF comes in handy in this type of question. Even if you do not recall by theory that the GCF cannot be greater than either terms, you can figure that out.
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Here's a question to further test your understanding of the concept of GCD:

If A and B are distinct positive integers greater than 1 such that the GCD of A and B is A, then which of the following must be true?

(A) A is a prime number (B) A and B have the same prime factors. (C) A and B have the same even-odd nature (D) All the factors of B are divisible by A (E) The LCM of A and B is B

Will post the solution in this thread on May 1, 2015. Till then, happy solving!

Which of the following CANNOT be the greatest common divisor [#permalink]

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27 Apr 2015, 11:56

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EgmatQuantExpert wrote:

Bunuel wrote:

Bumping for review and further discussion.

Hi Everyone!

Here's a question to further test your understanding of the concept of GCD:

If A and B are distinct positive integers greater than 1 such that the GCD of A and B is A, then which of the following must be true?

(A) A is a prime number (B) A and B have the same prime factors. (C) A and B have the same even-odd nature (D) All the factors of B are divisible by A (E) The LCM of A and B is B

Will post the solution in this thread on May 1, 2015. Till then, happy solving!

Regards Japinder

I think B, C and E are valid for the above question.
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Here's a question to further test your understanding of the concept of GCD:

If A and B are distinct positive integers greater than 1 such that the GCD of A and B is A, then which of the following must be true?

(A) A is a prime number (B) A and B have the same prime factors. (C) A and B have the same even-odd nature (D) All the factors of B are divisible by A (E) The LCM of A and B is B

Will post the solution in this thread on May 1, 2015. Till then, happy solving!

Regards Japinder

The correct answer is Option E.

PFB the correct solution for this question:

We are given that A and B are distinct positive integers greater than 1 such that the GCD of A and B is A

The important thing to note is that the question is asking about must be true statements. Must be true statements are those that will hold for all possible values of A and B, without exception.

So, our approach here will be to see if we can find any exceptions to the 5 given statements. Let's see.

(A) A is a prime number Consider A = 20 and B = 60. In this case, GCD(A,B) = A but A is not a prime number. Since we have found an exception to Statement A, it is clearly not a must be true statement.

(B) A and B have the same prime factors. Once again, consider the case of A = 20 and B= 60. The prime factors of A are 2 and 5. The prime factors of B are 2, 3 and 5. So, clearly Statement B doesn't hold true for all possible values of A and B, and therefore, cannot be a must be true statement.

(C) A and B have the same even-odd nature Consider A = 3 and B = 6. Here too, GCD(A,B) = A but the even-odd nature of A and B is opposite. So, Statement C is ruled out as well.

(D) All the factors of B are divisible by A In the case of A= 20 and B = 60, 15 is a factor of B that is not divisible by A. Similarly, in the case of A = 3 and B = 6, 1 is a factor of B that is not divisible by A

The existence of these exceptions indicates that Statement D is not a must be true statement.

(E) The LCM of A and B is B We know that LCM(A,B)*GCD(A,B) = A*B . . . (1)

Given: GCD(A,B) = A . . . (2)

On substituting (2) in (1), we get: LCM(A,B) = B

Therefore, Statement E will always be true, for all values of A and B.

Thank you for attempting this question. Please go through the solution posted above and let me know if you have any doubts about it.

See you around!

Japinder

Thanks Jaspinder...

I want to clarify two things which are what is the level of this question? and how to approach number system questions (I mean substituting values and working through is the best way to approach questions)?
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I want to clarify two things which are what is the level of this question? and how to approach number system questions (I mean substituting values and working through is the best way to approach questions)?

1. This question is of GMAT 650- difficulty level. That said, I don't think the difficulty-level of a question is an important number. During the preparation stage, one should focus on the learning that one can glean from a question. And, every question that can teach you something - whether a conceptual point or a takeaway on how to attempt questions better - is an important question. By focusing in this manner on

i) building concepts ii) learning to solve questions methodically in a step-by-step manner iii) learning from the mistakes that one makes along the way

even the questions of the GMAT 700+ difficulty level will start seeming easy to you.

2. I am not too big a fan of solving questions by substituting numbers. This approach certainly appears appealing at the first look because it seemingly allows you to bypass conceptual understanding. And precisely there lies the problem with this approach - if, during your preparation, you solve questions by substituting numbers, you're depriving yourself of an opportunity to hone your conceptual understanding.

I always advise my students to work through questions from the first principles.

Since your question was specifically about Number Properties, I can actually share with you a tangible sample of what I mean:

Our Number Properties Live Classroom session is a free session and likewise, its recording too is freely accessible by all. Please click here to go to the recording (the video takes about 45 seconds to load). The Number Properties part begins from the 20th minute onwards. In this session, you'll find both basic and very advanced questions from Even-Odd numbers, Prime Numbers and LCM-GCD. And, you'll see for yourself how even the most difficult Number Properties questions can be solved by applying, in a step-by-step manner, the basic concepts that you already know.

I hope you found this discussion useful. Please let me know if I can be of any further help

Re: Which of the following CANNOT be the greatest common divisor [#permalink]

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29 Apr 2017, 17:41

Lets male the cases of all options => Option 1 => GCD(3,5)=1 Option 2 => GCD(3,3)=3=> x Option 3 => GCD(3,3)=3=>y Option 4 => GCD(2,1)=>1=>x-y Option 5 =>GCD of two numbers can never be greater than either of them. So as x+y is greater than both x and y => It can never be the GCD.

Re: Which of the following CANNOT be the greatest common divisor [#permalink]

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17 Aug 2017, 06:45

Bunuel wrote:

Lolaergasheva wrote:

Which of the following cannot be the GCD of two positive integers x and y? a 1 b x c y d x-y e x+y

Divisor of a positive integer cannot be more than that integer (for example integer 4 doesn't have a divisor more than 4, the largest divisor it has is 4 itself), so greatest common divisor of two positive integers x and y can not be more than x or y.

Answer: E.

Bunuel Is this true only for positive integers because a negative integer can have divisors that are greater than the number.

Which of the following cannot be the GCD of two positive integers x and y? a 1 b x c y d x-y e x+y

Divisor of a positive integer cannot be more than that integer (for example integer 4 doesn't have a divisor more than 4, the largest divisor it has is 4 itself), so greatest common divisor of two positive integers x and y can not be more than x or y.

Answer: E.

Bunuel Is this true only for positive integers because a negative integer can have divisors that are greater than the number.

For example: -4 has divisors -1, -2, 2, -4, 4, 1

(-1, 4) (-2,2) (-4,1)

Is this understanding correct ?

Luckily you don't have to worry about that because every GMAT divisibility will tell you in advance that variables are positive integers only.
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Which of the following CANNOT be the greatest common divisor of two positive integers x and y?

A. 1 B. x C. y D. x-y E. x+y

Since the greatest common divisor or greatest common factor (GCF) of any two positive integers must be no larger than the lesser of the two integers, the GCF can’t be sum of the two integers. That is, the GCF of x and y can’t be x + y.

Answer: E
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