Problem Solving

pemdas wrote:

rijul007 wrote:FIX the value as 2??? why do you want to do that???

This question whether or not is up to the standards of GMAT, I dont know.
But even for the real GMAT, fixing a single number to a variable isnt a right thing to do.

if you don't have y=2 or any non-prime fraction consequently reduced in the denom. to 2, then you don't comply with

gmatrant wrote:If y not equals 3, and 2x/y is a prime integer greater than 2, which of the following must be true?
1. x=y
2. y=1
3. x and y are prime integers

and if you don't comply with the highlighted question text, then with what you should comply to solve this question?

The den does ultimately reduce to 2
but that does not mean y=2
y will still be 2/5 or 10 or 18 or whatever that it initially was.

I made it pretty much clear in my previous post.


I wont comment on this post anymore as i dont think there is anything fruitful coming out of this discussion.


I request the experts to kindly clarify this question and put an end to this mess.

i think you missed the previous page (edit now)

>>> say x=5/3 and y=2/3, consequently you have 5/3 : 2/3 = 5/2. Both 5/3 and 2/3 are non-primes (they are also non-integers) but MUST be reduced to 2 for 2x/y to be prime.

It MUST be true for x,y to be primes, given y is 2 (prime) and the values of x and y are primes. Other non-prime values of x and y MUST be true, given their ratio returns 2x/y as prime.

i'd also request experts to comment here

@Anurag, question states y is not equal to 3.
Question's condition (III) states y is prime. How to understand this? What's the point of including y as a prime in the condition (III) when it says y isn't equal to 3?

Shall we dissect condition (III) right away, without any processing it? Or we consider the condition (III) as one implying y not any prime?

If condition (III) doesn't imply any prime or all primes, then why we should consider y as 17 or 11 or any other prime?

Anurag@Gurome wrote:

gmatrant wrote:If y not equals 3, and 2x/y is a prime integer greater than 2, which of the following must be true?
1. x=y
2. y=1
3. x and y are prime integers

A) None
B) I only
C) II only
D) III only
E) I and II

1. x = y
So, 2x/y = 2y/y = 2, which is not greater than 2. So, (1) cannot be true.

2. y = 1 implies 2x/y = 2x
Now if x = 1, then 2x/y = 2, which is not greater than 2. So, (2) cannot be true always.

3. x and y are prime integers.
If x = 17 and y = 2, then 2x/y = 17, which is a prime integer greater than 2
If x = 2 and y = 17, then 2x/y = 4/17 = 0.2, which is not a prime integer.
It can be seen from the above examples that (3) is also not always true.

The correct answer is A.

aha, you are there
you now linked question with condition (III) by saying that y can be any other prime except for 3. Hence you must also say that 2x/y is prime, because you accepted y=!3 from the question and you must accept 2x/y must be prime. But here you contadict

If x = 2 and y = 17, then 2x/y = 4/17 = 0.2, which is not a prime integer.

why? to accept y isn't 3 but prime and not accept 2x/y is prime is like accepting part of the question for *some purpose* and not accepting the other part of the question. Please explain.

Anurag@Gurome wrote: y is not equal to 3 does not imply that y cannot be any other prime integer. It simply means that y can be any prime integer other than 3.

I am not saying answer A is not good. What I am saying is point in condition (III) and argumentation you and some other posters had with this condition. Shall we dissect condition (III) right away? Shall we say that x,y are primes is bad condition for this question as with the given y=!3 this condition will not hold true ever?

revised

Ian Stewart wrote:... It asks if III must be true if 2x/y is prime.

If what Anurag says is completely backwards and we agree that 2x/y is true (obviously, as this proceeds from the question), please show me any prime values for x and y which taken with the prerequisite (2x/y is prime) are not true. All prime values for x,y are true and the question can be answered Yes, x,y must be prime for 2x/y to be prime.

The other question is CAN x,y be non-prime? We answer to this question Yes if we are asked about "CAN BE" possibility and answer No if this is "MUST BE" condition. But this is another question ... The only "MUST BE" condition for the original question which works 100% all time with x,y is when they are prime. For non-prime values of x,y this is certainty (possibility) but not necessity.

Ian, I am still having my doubt, as you showed that x,y can be non-primes, but this doesn't mean that when x,y "MUST BE" primes their not always giving the required 2x/y prime number.

Thanks, Ian!
I am having serious problems with question interpretation now. Most of incorrect answers I've marked recently on the forum proceeded from my wrong comprehension of the question(s) asked. Is there any other way I may practice question interpretation for GMAT?

I have also searched the forum archives and found this article by Knewton GMAT Prep https://www.beatthegmat.com/mba/2011/10/ ... -questions
a small excerpt from the article

So the big takeaway here is: On "must be true" questions, just one example of "false" is enough to demonstrate that a statement does not have to be true. But just one example of "true" is not enough to show that the statement must be true in all cases.

Hey here's my logic:
x=y; if this be true then 2x/y = 2 and 2 is not greater than itself.
y=1; if this be true, 2x/y = 2x and so 2x cannot be prime greater than 2, since it already has a factor of 2.
x and y are primes; if this be true, then there would be no factors common to x and y and hence 2x/y would be non integers and can not qualify to be a prime no.

Thanks
Abdul