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Manager

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If y ≠ 3 and 3x/y is a prime integer greater than 2, [#permalink ]
26 Nov 2007, 14:57

If y ≠ 3 and 3x/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 Ⅲ

CEO

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B.
1. if x<y> 3*n - is not a prime. so x=y. suff.
2. for (1,1) y=1 but for (2,2) y=2 insuff.
3. for (2,2) x,y - prime but for (4,4) x,y - not prime. insuff.

CEO

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Skewed wrote:

If y ≠ 3 and 3x/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 Ⅲ

must be!!! hmmm.................

ok, then it is B. I only because if x and y are cancelled out then only 3x/y remains a prime.

Manager

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What about [(5/3),1] for x,y in case I?
[3*(5/3)]/1= 5
A.

CEO

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Skewed wrote:

What about [(5/3),1] for x,y in case I? [3*(5/3)]/1= 5 A.

thats why the question says that y is not equal to 3.

though thats a good point.

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Intern

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Skewed wrote:

What about [(5/3),1] for x,y in case I? [3*(5/3)]/1= 5 A.

In case I, Y has to be equal to X.

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Skewed wrote:

What about [(5/3),1] for x,y in case I? [3*(5/3)]/1= 5 A.

Thats pretty good.

Tiger he says that (3*5/3)/1 X=5/3. So A looks good.

Director

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B it is,
was going to say E..

Director

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GMATBLACKBELT wrote:

Skewed wrote:

What about [(5/3),1] for x,y in case I? [3*(5/3)]/1= 5 A.

Thats pretty good.

Tiger he says that (3*5/3)/1 X=5/3. So A looks good.

from the question:

3x=y*prime number-----> either y=3 or prime number=3

y is not 3, thus Prime number= 3----> 3x=y3---->x=y, where x and y are not necessarily prime numbers.

Manager

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Senior Manager

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Skewed wrote:

What about [(5/3),1] for x,y in case I? [3*(5/3)]/1= 5 A.

Good point was going for B..

Director

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Skewed wrote:

What about [(5/3),1] for x,y in case I? [3*(5/3)]/1= 5 A.

it was not mentioned that x is an intenger...missed this point

vote for A

Senior Manager

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agh! the simple ones...
how do one avoid felling for these tricks?

Director

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asdert wrote:

agh! the simple ones... how do one avoid felling for these tricks?

"assumed to be possible unless stated or proved otherwise"

unless it says that a number is not a ratio, same as to say that a number is an intenger (as in the example above), we can correctly assume that the number is a ratio ..

simple indeed,

i fell for too

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It is A.
x=1/3
Y=1/5
3x/y = 5 which is prime
so x != y

SVP

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If y ≠ 3 and 3x/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
for 3x/y to be prime , x/y must = 1
i believe it is A

SVP

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did we get a final consensus on this one ?
I am getting A. Here is why.
Q basically tells us that 3x/y =3,5,7,9, ...
1 says x must equal y. not true, because if 3x/y=5, we just get 3x=5y. not always true
2 says y must be 1. again, from above, we can see it doesnt have to be the case.
3. x and y must be prime. i dont think they do, just by looking at the above.
so since none fit the bill, i went with A

Manager

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it is obviously A:
Pick numbers: x=14, y=6
3*14/6=7. None of the choices works. And no restrictions on x=14 and y=6