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# A set of numbers has the property that for any number t in

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Director
Joined: 25 Oct 2006
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A set of numbers has the property that for any number t in [#permalink]

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30 May 2007, 09:30
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

1. 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

-------------------
2. S is a set of integers such that
i) if a is in S, then –a is in S, and
ii) if each of a and b is in S, then ab is in S.
Is –4 in S?

(1) 1 is in S.
(2) 2 is in S.

I chalked out the first one as E, because,
if t = -1; t+2 = 1 (B) and again t+2 = 3 and so on we'll get 5(C)
Again if t+2 = -1 then t = -3 (A)

For 2 it's B.
Manager
Joined: 25 May 2007
Posts: 230
Schools: Cornell
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Re: Anybody has Solution for these?? [#permalink]

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30 May 2007, 09:50
1. E

If -1 is in the set, then the set is ...-7,-5,-3,-1,3,5,7,..., that is all the odd numbers.

2. B

(1) a = -1, then b = 1, ab = -1, can't tell anything about -4
(2) a = 2, then b = -2, then ab = -4. This is sufficient.
Intern
Joined: 04 Feb 2007
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30 May 2007, 11:23
1. D for if (-1) is the first no. of the set, then (-3) MUST NOT be there.

2. B, AGREED.
VP
Joined: 08 Jun 2005
Posts: 1145
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30 May 2007, 13:26
1. 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

Don't assume that -1 is the first on the set (the stem doesn't say that).

we can say from the stem that "for any number t in the set, t + 2 is in the set" but I think we can't say that "for any number t in the set, t - 2 is in the set" becuase once again it's an assumption !! the GMAT don't want us to assume on PR problems.

so, if -1 is in the set , 1 must be in the set, if 1 is in the set 3 must be in the set and so do 5 ! (but not -3).

Director
Joined: 25 Oct 2006
Posts: 640
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Kudos [?]: 554 [0], given: 6

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30 May 2007, 18:34
Thanks, KillerSquirrel
Now it's clear.
Director
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Re: Anybody has Solution for these?? [#permalink]

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30 May 2007, 18:53
priyankur_saha@ml.com wrote:
1. 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

-------------------
2. S is a set of integers such that
i) if a is in S, then –a is in S, and
ii) if each of a and b is in S, then ab is in S.
Is –4 in S?

(1) 1 is in S.
(2) 2 is in S.

I chalked out the first one as E, because,
if t = -1; t+2 = 1 (B) and again t+2 = 3 and so on we'll get 5(C)
Again if t+2 = -1 then t = -3 (A)

For 2 it's B.

Answer should be E and B respectively. What did you not understand?
Director
Joined: 25 Oct 2006
Posts: 640
Followers: 14

Kudos [?]: 554 [0], given: 6

Re: Anybody has Solution for these?? [#permalink]

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31 May 2007, 11:36
LM wrote:
priyankur_saha@ml.com wrote:
1. 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

-------------------
2. S is a set of integers such that
i) if a is in S, then –a is in S, and
ii) if each of a and b is in S, then ab is in S.
Is –4 in S?

(1) 1 is in S.
(2) 2 is in S.

I chalked out the first one as E, because,
if t = -1; t+2 = 1 (B) and again t+2 = 3 and so on we'll get 5(C)
Again if t+2 = -1 then t = -3 (A)

For 2 it's B.

Answer should be E and B respectively. What did you not understand?

See this Explanation above
Don't assume that -1 is the first on the set (the stem doesn't say that).

we can say from the stem that "for any number t in the set, t + 2 is in the set" but I think we can't say that "for any number t in the set, t - 2 is in the set" becuase once again it's an assumption !! the GMAT don't want us to assume on PR problems.

so, if -1 is in the set , 1 must be in the set, if 1 is in the set 3 must be in the set and so do 5 ! (but not -3).

I assumed -1 as t+1 and got tha answer choice A also true. Based on the above explanation I shouldn't. Right??
VP
Joined: 08 Jun 2005
Posts: 1145
Followers: 7

Kudos [?]: 212 [0], given: 0

Re: Anybody has Solution for these?? [#permalink]

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31 May 2007, 11:56
priyankur_saha@ml.com wrote:
LM wrote:
priyankur_saha@ml.com wrote:
1. 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

-------------------
2. S is a set of integers such that
i) if a is in S, then –a is in S, and
ii) if each of a and b is in S, then ab is in S.
Is –4 in S?

(1) 1 is in S.
(2) 2 is in S.

I chalked out the first one as E, because,
if t = -1; t+2 = 1 (B) and again t+2 = 3 and so on we'll get 5(C)
Again if t+2 = -1 then t = -3 (A)

For 2 it's B.

Answer should be E and B respectively. What did you not understand?

See this Explanation above
Don't assume that -1 is the first on the set (the stem doesn't say that).

we can say from the stem that "for any number t in the set, t + 2 is in the set" but I think we can't say that "for any number t in the set, t - 2 is in the set" becuase once again it's an assumption !! the GMAT don't want us to assume on PR problems.

so, if -1 is in the set , 1 must be in the set, if 1 is in the set 3 must be in the set and so do 5 ! (but not -3).

I assumed -1 as t+1 and got tha answer choice A also true. Based on the above explanation I shouldn't. Right??

From my understanding of the question, you can't appoint (t+2) as -1. every organ in the set is t, so if (t+2) is in the set (t+2)+2 must be in the set but not (t+2)-2.

whats the OA ?
Director
Joined: 25 Oct 2006
Posts: 640
Followers: 14

Kudos [?]: 554 [0], given: 6

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31 May 2007, 11:58
KillerSquirrel, that's fine.

OA is D also.

Thank you.
31 May 2007, 11:58
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