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Consecutive odd

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Consecutive odd

by Uri » Mon Apr 20, 2009 8:39 am
If S = {2, 3, x, y}, what is the value of x + y?
(1) x and y are prime numbers.
(2) 3, x, and y are consecutive odd


Does statement (2) mean that 3, x and y are consecutive odd integers in the given order, i.e., 3<x<y?

This is a problem from GMAT Plus Book. And I made a mistake perhaps due to a wrong assumption. Please help me clarify my doubt, before you look at the OA. Since I don't think that all the answers given in GMAT Plus Book are trustworthy, I am asking this question.

OA: [spoiler](B)[/spoiler]
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by DanaJ » Mon Apr 20, 2009 9:00 am
To answer your question: yes, it does. The immediately greater odd numbers will be 5 and 7, so x + y can be easily determined.

Solving the problem:

1. x could be 5 and y could be 23
x could be 29 and y could be 7

There is no way of picking a single pair of prime numbers, since there are sooo many...This is why 1 is insufficient.

2. is sufficient since the odd numbers you are looking for will be 5 and 7.
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by austin » Tue Apr 21, 2009 4:02 am
If S = {2, 3, x, y}, what is the value of x + y?
(1) x and y are prime numbers.
(2) 3, x, and y are consecutive odd

statement 1: x and y are prime, could be any values
statement 2: TRICKY!!!
3, x, and y are consecutive odd => x = 1, then comes 3 then y = 5
OR
3, x, and y are consecutive odd => 3, then x = 5, then y = 7

value of x+y is different.. (IT IS NOT GIVEN THAT y>x>3)

COMBINE: we get 3,5,7 (remember: 1 is NOT prime)
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by ketkoag » Tue Apr 21, 2009 11:59 am
it is not mentioned in statement 2 that in which order they are consecutives. so i don't think that statement 2 is sufficient.
IMO C.
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by Musiq » Tue Apr 21, 2009 1:18 pm
Uri, Odd and Even numbers are defined for negative numbers also.

So we absolutely need both statements to answer the question.

C is my answer too.

As an aside, the GMAT DOES NOT take into account negative factors....even though they exist.

But I have not seen any express citations from GMAC that say "We only imply non-negative when talking Odd/Even".

So, even test-philosophy wise I would say C is the answer.
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by DanaJ » Tue Apr 21, 2009 1:28 pm
IMHO you are over-analyzing this. When I see 3, x and y consecutive odds I immediately think that they're just like that: 3 < x < y.

Wondering about the order just seems too "picky" for me; after all, this is about logic and maths, not looking for the really small things (as the French might put it, vous cherchez la petite bete)

Actually, when you see smth like "a, b and c are consecutive numbers", do you wonder if it's a < b < c or b < a < c? I think not; you just go with the flow and assume that a < b < c. It's the same case here.

Again, this is my personal opinion. Some might look at it differently.
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by Musiq » Tue Apr 21, 2009 1:47 pm
DanaJ wrote:IMHO you are over-analyzing this. When I see 3, x and y consecutive odds I immediately think that they're just like that: 3 < x < y.

Wondering about the order just seems too "picky" for me; after all, this is about logic and maths, not looking for the really small things (as the French might put it, vous cherchez la petite bete)

Actually, when you see smth like "a, b and c are consecutive numbers", do you wonder if it's a < b < c or b < a < c? I think not; you just go with the flow and assume that a < b < c. It's the same case here.

Again, this is my personal opinion. Some might look at it differently.
Let's for a moment assume that Statement 2 gives us a set that looks like {2,3,5,7}......and not {2,3,1,-1}.

Then why is Statement 1 talking about X and Y being prime numbers, which are by definition >1?

It is not at all GMAT like to give a random Statement like that without there being some rhyme or reason (either a trap to make the test-taker think there is sufficiency, or to use in conjunction to get C)
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by cramya » Tue Apr 21, 2009 1:51 pm
I think the responses above are correct in their respective ways. If the OA is B then the question is slightly ambigous in terms of what they framer of the question wanted use to assume 3<x<y

The problem could have been more clear by saying "S is an ordered set" or something like that to safely conclude 3<x<y. Just my opinion. Like DanaJ said the interpretation differs from person to person. Sometimes assumptions like these can bite us in the back i.e. between a C and E choice just as an example.


Please use this carefully:

When I looked at statement 2 I could have taken a cue(nothing more since the statements need to be evaluated independently first) from stmt I that the question writer did not want us to take x or y to be 1 since 1 is not prime.

The 2 statements in a DS never contradict each other.

x or y cant be prime in I and not prime in II

Again, the question could have been written a little more clearly where there is no room for ambiguity discussions.

Regards,
CR
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