is ans is correct????

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arorag: Master | Next Rank: 500 Posts; Posts: 343; Joined: Mon Jan 21, 2008 3:28 pm; Thanked: 4 times

by arorag » Mon Sep 01, 2008 11:22 am

A set of numbers has the property that for any number t in the set, t + 2 is in the set. If –1
is in the set, which of the following must also be in the set?
I. -3
II. 1
III. 5
A. I only
B. II only
C. I and II only
D. II and III only
E. I, II, and III

Ans D

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Source: Beat The GMAT — Problem Solving | Mon Sep 01, 2008 11:22 am

sudhir3127: Legendary Member; Posts: 829; Joined: Mon Jul 07, 2008 10:09 pmFollowed by: 3 members; Location: INDIA; Thanked: 84 times

Re: is ans is correct????

by sudhir3127 » Mon Sep 01, 2008 11:50 am

arorag wrote:A set of numbers has the property that for any number t in the set, t + 2 is in the set. If –1
is in the set, which of the following must also be in the set?
I. -3
II. 1
III. 5
A. I only
B. II only
C. I and II only
D. II and III only
E. I, II, and III

Ans D

i will go with D as well

it tells us that for every t t+2 will be there ,..
so be the set contains -1

-1 (+2 ) on a number line will be +1
and u will also have 3 ,5,7...............

there is a chance u may or may not have -3

this is a trap answer...

"we can say -3 is there that the reason -1 is there" which is logically correct and holds good according to the Question... but what if

the set starts from -1 ..then we wont have -3... so its a trap answer...

hope this helps...

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sushilmore: Junior | Next Rank: 30 Posts; Posts: 16; Joined: Mon Dec 10, 2007 8:53 pm

by sushilmore » Mon Sep 01, 2008 2:57 pm

Hi Sudhir,

They have not mentioned that set starts from -1. It just mentions that -1 is part of the set. So I think -3 should be part of the set and hence E

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GMAT Genie: Newbie | Next Rank: 10 Posts; Posts: 5; Joined: Sun Aug 31, 2008 11:00 pm; Location: Sydney

Answer is E

by GMAT Genie » Mon Sep 01, 2008 11:34 pm

i will go with E as -3 can be in the set and there is no starting point of the set

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GMAT Genie: Newbie | Next Rank: 10 Posts; Posts: 5; Joined: Sun Aug 31, 2008 11:00 pm; Location: Sydney

Answer is E

by GMAT Genie » Mon Sep 01, 2008 11:36 pm

i will go with E as -3 can be in the set and there is no starting point of the set

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Ian Stewart: GMAT Instructor; Posts: 2623; Joined: Mon Jun 02, 2008 3:17 amFollowed by: 355 members; Location: Montreal; Thanked: 1090 times; GMAT Score:780

by Ian Stewart » Tue Sep 02, 2008 3:11 am

-3 certainly can be in the set, but that's not what the question asks. The question asks whether -3 must be in the set. And -3 does not need to be in the set; the set could be:

{-1, 1, 3, 5, 7, ...}

Then if you pick any number t in the set, t+2 is also in the set, so this set satisfies the conditions given in the question. The answer is D.

Last edited by Ian Stewart on Tue Sep 02, 2008 5:56 pm, edited 1 time in total.

For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

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GMAT Genie: Newbie | Next Rank: 10 Posts; Posts: 5; Joined: Sun Aug 31, 2008 11:00 pm; Location: Sydney

by GMAT Genie » Tue Sep 02, 2008 4:14 pm

My understanding is that if there is t in the set, then t+2 must be in the set.
Since the question is saying that -1 is in the set, that means the -3 is definitley present in the set to allow -1 in the set. I may be wrong but thats my interpretation

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GMAT Genie: Newbie | Next Rank: 10 Posts; Posts: 5; Joined: Sun Aug 31, 2008 11:00 pm; Location: Sydney

by GMAT Genie » Tue Sep 02, 2008 4:24 pm

I again read the question and I conclude that D is more appropriate. The trick in the question is may be and must be. Answer D is more appropriate for 'Must be' part of the question

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