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If y ≠ 3 and 2x/y is a prime integer greater than 2, which

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If y ≠ 3 and 2x/y is a prime integer greater than 2, which [#permalink] New post 13 Dec 2010, 16:41
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If y ≠ 3 and 2x/y is a prime integer greater than 2, which of the following must be true?

I. x = y
II. y = 1
III. x and y are prime integers.

(A) None
(B) I only
(C) II only
(D) III only
(E) I and II
[Reveal] Spoiler: OA

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Re: Inequality - Prime number [#permalink] New post 13 Dec 2010, 16:44
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Let 2x/Y = K , Where k is a prime number > 2 ( 3, 5, 7, 11 ...etc)

so 2x = Ky --> ( 3y or 5y or 7y or 11y etc...)

Which implies alwasy X > Y


So answer is (A)
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Re: Inequality - Prime number [#permalink] New post 13 Dec 2010, 17:08
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saurabhgoel wrote:
35. If y ≠ 3 and 2x/y is a prime integer greater than 2, which of the following must be true?
I. x = y
II. y = 1
III. x and y are prime integers.

(A) None

(B) I only

(C) II only

(D) III only

(E) I and II


I dont know the OA ? So please help ?
My pick is (A)


Note that we are asked "which of the following MUST be true, not COULD be true. For such kind of questions if you can prove that a statement is NOT true for one particular set of numbers, it will mean that this statement is not always true and hence not a correct answer.

So we should try to make the listed statements not true for some x and y (which satisfy y≠3 and 2x/y=prime>2).

I. x = y --> not true: x=3 and y=2 (2x/y=3=prime>2);

II. y=1 --> not necessarily true: x=3 and y=2 (2x/y=3=prime>2);

III. x and y are prime integers --> not necessarily true: x=10 and y=4 (2x/y=5=prime>2).

Answer: A.

Similar question: ps-number-property-77261.html
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Re: Inequality - Prime number [#permalink] New post 10 Mar 2011, 18:01
Given 2x/y is prime >2

could be {3,5,7,11....}

I) x = y => 2x/y = 2 , 2 will never be prime >2 (Ruled out)
II)y=1 . Not necessarily. 2 (3)/2 = 3 which is prime and here y is 2 (Ruled out)
III)x and y are prime integers . not necessarily . (Ruled out)
2(3)/2 x and y prime
2(21)/6 x and y are not prime.

Answer is A (None)
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Re: Inequality - Prime number [#permalink] New post 11 Mar 2011, 23:09
Bunuel wrote:
saurabhgoel wrote:
35. If y ≠ 3 and 2x/y is a prime integer greater than 2, which of the following must be true?
I. x = y
II. y = 1
III. x and y are prime integers.

(A) None

(B) I only

(C) II only

(D) III only

(E) I and II


I dont know the OA ? So please help ?
My pick is (A)


Note that we are asked "which of the following MUST be true, not COULD be true. For such kind of questions if you can prove that a statement is NOT true for one particular set of numbers, it will mean that this statement is not always true and hence not a correct answer.

So we should try to make the listed statements not true for some x and y (which satisfy y≠3 and 2x/y=prime>2).

I. x = y --> not necessarily true: x=3 and y=2 (2x/y=3=prime>2);

II. y=1 --> not necessarily true: x=3 and y=2 (2x/y=3=prime>2);

III. x and y are prime integers --> not necessarily true: x=10 and y=4 (2x/y=5=prime>2).

Answer: A.


Dear bunuel
can't we go have another approach
by considering the options from 1st to 3rd
and see if 2x/y is not a prime no >2
as below
I. x = y --> then 2x/y=2 which is not greater then 2 so can't be true
II. y=1 --> then 2x/y=2 which is also not greater then 2 so can't be true
III. x and y are prime integers --> then this value will be fractional only so cant be an INTEGER so can't be true

hence A is the answer

please enlighten me for this approach :)

Regards
Puneet
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Re: Inequality - Prime number [#permalink] New post 11 Mar 2011, 23:12
Bunuel wrote:
saurabhgoel wrote:
35. If y ≠ 3 and 2x/y is a prime integer greater than 2, which of the following must be true?
I. x = y
II. y = 1
III. x and y are prime integers.

(A) None

(B) I only

(C) II only

(D) III only

(E) I and II


I dont know the OA ? So please help ?
My pick is (A)


Note that we are asked "which of the following MUST be true, not COULD be true. For such kind of questions if you can prove that a statement is NOT true for one particular set of numbers, it will mean that this statement is not always true and hence not a correct answer.

So we should try to make the listed statements not true for some x and y (which satisfy y≠3 and 2x/y=prime>2).

I. x = y --> not necessarily true: x=3 and y=2 (2x/y=3=prime>2);

II. y=1 --> not necessarily true: x=3 and y=2 (2x/y=3=prime>2);

III. x and y are prime integers --> not necessarily true: x=10 and y=4 (2x/y=5=prime>2).

Answer: A.



Dear bunuel
can't we go have another approach
by considering the options from 1st to 3rd
and see if 2x/y is not a prime no >2
as below
I. x = y --> then 2x/y=2 which is not greater then 2 so can't be true
II. y=1 --> then 2x/y=2x which is an even number always so can't be true
III. x and y are prime integers --> then this value will be fractional only so cant be an INTEGER so can't be true

hence A is the answer

please enlighten me for this approach :)

Regards
Puneet
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see my Test exp here http://gmatclub.com/forum/my-test-experience-111610.html
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If y ≠ 3 and 2x/y is a prime integer greater than 2, which of th [#permalink] New post 10 Jan 2013, 18:52
'A' is the right option. Condition III is satisfied when y=2 and x= some prime integer, but for other primes, it negates the condition. Hence 'None'.
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If y ≠ 3 and 2x/y is a prime integer greater than 2, which of th   [#permalink] 10 Jan 2013, 18:52
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