integers

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ketkoag: Legendary Member; Posts: 876; Joined: Thu Apr 10, 2008 8:14 am; Thanked: 13 times

integers

by ketkoag » Thu Jun 11, 2009 4:03 am

Integers 9, 11, x, y, 17 are consecutive. Y=?
1) x is odd
2) y is odd

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Source: Beat The GMAT — Data Sufficiency | Thu Jun 11, 2009 4:03 am

Stuart@KaplanGMAT: GMAT Instructor; Posts: 3225; Joined: Tue Jan 08, 2008 2:40 pm; Location: Toronto; Thanked: 1710 times; Followed by:614 members; GMAT Score:800

Re: integers

by Stuart@KaplanGMAT » Thu Jun 11, 2009 10:23 am

ketkoag wrote:Integers 9, 11, x, y, 17 are consecutive. Y=?
1) x is odd
2) y is odd

Hi!

What's the source of this question? Normally when we discuss "consecutive integers", we specifically mean sets such as {2, 3, 4, 5, 6}.

The word "consecutive" doesn't always mean "consecutive integers"; for example, we could discuss "consecutive odd numbers" or "consecutive multiples or 4".

That said, the only way this question makes sense is if our set is {9, 11, 13, 15, 17} and we are in fact looking at consecutive odd integers.

So, the question really is:

If x and y make up the set {13, 15}, which is which?

(1) x is odd. Could be 13 or 15... insufficient.

(2) y is odd. Could be 13 or 15... insufficient.

Combining: still no clue which is 13 and which is 15: insufficient, choose (E).

Stuart Kovinsky | Kaplan GMAT Faculty | Toronto

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david2008: Senior | Next Rank: 100 Posts; Posts: 31; Joined: Sat May 09, 2009 7:45 pm; Location: Bay Area; Thanked: 4 times

by david2008 » Thu Jun 11, 2009 12:37 pm

What is the OA?

It would have to be E if we can't assume the set is written in order but if it is written in order then shouldn't it be C?

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Stuart@KaplanGMAT: GMAT Instructor; Posts: 3225; Joined: Tue Jan 08, 2008 2:40 pm; Location: Toronto; Thanked: 1710 times; Followed by:614 members; GMAT Score:800

by Stuart@KaplanGMAT » Thu Jun 11, 2009 1:28 pm

david2008 wrote:What is the OA?

It would have to be E if we can't assume the set is written in order but if it is written in order then shouldn't it be C?

If it's written in order, then we can answer without the statements; therefore, it can't be written in order.

In data sufficiency, the original information itself will never be sufficient to answer the question (there's no answer choice that says "the question can be answered without any additional information").

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ket: Senior | Next Rank: 100 Posts; Posts: 93; Joined: Sun Mar 15, 2009 9:07 am; Thanked: 7 times

by ket » Fri Jun 12, 2009 2:44 am

IMHO I think it is C

if I were solving this problem on exam I would understand the problem as as - 9<11<x<y<17... i.e. numbers are placed in order...

(1) - could be 13 or 15
(2) - could be 13 or 15

combined it y must be 15 because x<y

If I am correct Stuart thinks the problem asks for 'consecutive odd integers' and plus assumes that the numbers are not placed in the way of order... Is this assumption justified? I mean the problem doesn't really mention the word 'odd' , while I think that assumption that values are placed in order is justified...

I think so well because, if the problem was written something like this: the Set (9,11,x,y,17) contains consecutive integers... then I would not assume the order... but the way the question is put I think the order is assumed....

It would be great to hear OA from Ketkoag

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bizwizashish: Newbie | Next Rank: 10 Posts; Posts: 6; Joined: Wed Feb 14, 2007 1:34 am; Location: India, bangalore

by bizwizashish » Sat Jun 13, 2009 8:54 am

OA ?

I guess it's fair to assume that numbers are written in order.... atleast the wording suggests so...

would really be interested in knowing the OA

Regards,
Ashish Khandelwal

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The GMAT Chef: Junior | Next Rank: 30 Posts; Posts: 18; Joined: Sat May 16, 2009 2:11 pm; Location: San Francisco Bay Area

Re: integers

by The GMAT Chef » Sat Jun 13, 2009 11:06 am

ketkoag wrote:Integers 9, 11, x, y, 17 are consecutive. Y=?
1) x is odd
2) y is odd

Hi,
Wherever this question is from, it's not of the GMAT type.

Two issues

First issue:

As Stuart pointed out, when the term "consecutive integers" is used, it is understood that the integers are consecutive by 1. Otherwise, we say consecutive odd, consecutive even, consecutive by ten, etc...

Consecutive integers are just an example of an arithmetic progression (sequence) in which the constant R (the difference between two consecutive terms) is equal to 1.
For consecutive odd integers, R = 2.
For consecutive even integers, R = 2 as well.

So, the question will only make sense if the integers are consecutive odd.
But then, we don't need statements 1 or 2 and the question does no longer qualify as a GMAT DS question.

Second issue.

In the stem,

a)If we were told that the list was ordered, we could answer the questions without the statements: y = 15

b)Likewise, if we were told that the list was unordered, we could answer the questions without the statements: y = 13

Neither case is acceptable in the GMAT DS format as Suart judiciously pointed out.

The real issue is not knowing whether the list is ordered.

For this reason, the correct answer could be any choice depending upon what the statements are.

This may be trivial but a pair of statements could be:
1) the integers are not necessarily in order
2) x - y = -2

Answer: B (y= 15)

Dakar Azu is The GMAT Chef. He has sweet recipes for virtually every type of GMAT question, be it quantitative or verbal. Dakar has been teaching the GMAT since 2003 and is the founder of GMATLounge at https://gmatlounge.com , 700-GMAT Club at https://700gmatclub.com ,
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