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2x/y = prime

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by caspermonday » Sun Oct 25, 2009 8:04 am
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

No OA again. I am hesitating btw a and d...
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by gmatv09 » Sun Oct 25, 2009 8:27 am
IMO A
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by caspermonday » Sun Oct 25, 2009 9:30 am
Do you solve III by plugging numbers or how?
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by xcusemeplz2009 » Sun Oct 25, 2009 9:36 am
IMO D

the expression 2x/y can be prime > 2 only when y=2 and x is any prime no.>2

hence bth x and y must be a prime no.
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by gmatv09 » Sun Oct 25, 2009 9:39 am
sorry agree with xcusemeplz2009 ...
I was confused :-(

2x/y can be a prime no. only if y = 2
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by satish.nagdev » Sun Oct 25, 2009 10:26 am
IMO A,
if x=5 and y = 7, 2x/y is not a integer at first place
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by xcusemeplz2009 » Sun Oct 25, 2009 10:36 am
satish.nagdev wrote:IMO A,
if x=5 and y = 7, 2x/y is not a integer at first place
yeah very true but the Q demands when 2x/Y is a prime no. then what must be true....so we need to think of a case where 2x/y is a prime no.
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by sudeeparies » Sun Oct 25, 2009 6:05 pm
i agree its D
only if y=2 can this expression be a integer and prime for any value of x (prime again)
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by satish.nagdev » Sun Oct 25, 2009 6:08 pm
xcusemeplz2009 wrote:
satish.nagdev wrote:IMO A,
if x=5 and y = 7, 2x/y is not a integer at first place
yeah very true but the Q demands when 2x/Y is a prime no. then what must be true....so we need to think of a case where 2x/y is a prime no.
oh man yea my bad
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by pandeyvineet24 » Sun Oct 25, 2009 8:22 pm
satish.nagdev wrote:
xcusemeplz2009 wrote:
satish.nagdev wrote:IMO A,
if x=5 and y = 7, 2x/y is not a integer at first place
yeah very true but the Q demands when 2x/Y is a prime no. then what must be true....so we need to think of a case where 2x/y is a prime no.
oh man yea my bad
actually i think it should be A.

(2*x )/y should be prime

plug x = 21 and y = 6, the expression reduces to 7, which is a prime number. Therefore x,y need not be prime.
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by sanjana » Sun Oct 25, 2009 9:14 pm
IMO : A

I. x = y
If X=Y then 2X/Y = 2 which i primt but not greater than 2. Hence,rule out B and E
II. y = 1
If y=1,then 2x will be 2(which is not greater than 2) if x=1 and if it is anything else other than 1 it will not result in a prime number as E*ANYTHING = EVEN and 2 is the only even prime

III. x and y are prime integers.
If x and y are prime integers,then 2x/y cannot be an integer unless x=y(in this case the anser will be 2,not acceptable) .For any other combination it wont be greater than 2.

Hence D.
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Re: 2x/y = prime

by Stuart@KaplanGMAT » Mon Oct 26, 2009 1:25 pm
caspermonday wrote: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
The answer is A (none).

We know that 2x/y is a prime greater than 2. Therefore, x/y must be greater than 1, so eliminate (I).

We can quickly eliminate II and III by picking numbers. If we set 2x/y = 3 (a prime number greater than 2), we get:

x/y = 3/2

So, any two numbers in a ratio of 3:2 will suffice. We can let x=6 and y=4 to see that neither statement II nor III (nor I, for that matter) must be true.

As an aside, although it wasn't necessary for this question, we should remember to be careful about the assumptions we make. Nowhere in the question stem does it say that x and y have to be integers - we certainly could have picked non-integer values to eliminate all 3 statements as well.
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by caspermonday » Mon Oct 26, 2009 10:16 pm
Thank you, Stuart! Now we are sure :)
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by vivek.kapoor83 » Tue Oct 27, 2009 3:23 am
ultimate o bekctive is to find the 2x/y as prime
by p;lugging x =14, y=4 we get 2x/y = 7 which is prime no greater than 2.
so, A is clear winner in 10 sec..
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by hongwang9703 » Tue Oct 27, 2009 6:52 pm
stuart is like the litigator, who gets to the bottom of things lol reading this thread, i changed the answer 3 or 4 times
i got utterly defeated by the gmat.
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