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

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12 Apr 2005, 09:30

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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

I was thinking B first. but, what does "A set of numbers has the property that for any number t in the set, t + 2 is in the set"?

i think any number indicates to an infinite series. if so, it should be E.
t could be any number in the series. if t is in the series, t+2 is also in the series. if t=t+2, then t+2=t+4. then if t+2=t, then t=t-2.

Question is 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?

Doubt 1 - Lets say t= -1 ; then t+2 is is set = i.e. 1 is in set.

Since question stem is Must be True, how do we make sure that this pattern will continue. i.e if 1 is in set, then 3 will be in set and so on.. Also

Doubt 2 - Cant we assume that t+2 = -1 then t= -3(should also be in set)

Please clarify.

Thanks

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

The question is which of the following must be in the set, not could be in the set.

If -1 is in the set so must be -1+2=1, as 1 is in the set so must be 1+2=3, as 3 is in the set so must be 3+2=5 and so on. So basically knowing that -1 is in the set we can say that ALL odd numbers more than -1 are also in the set.

Answer: D.

Now, about your questions: 1. The question says "a set of numbers has the property that for ANY number t in the set, t + 2 is in the set", so if 1 is in the set then 1+2=3 MUST be in the set.

2. We don't know which is the source integer in the set, if it's -1 than odd number less than it won't be in the set but if source integer is let's say -11 than -3 will be in the set. So -3 may or may not be in the set.

D for me. We know for sure that t (-1) is in the set so t+2=+1 is in the set so 1+2=3 is in the set so 3+2=5 is in the set

-1 could be the first element of the set, so -3 can't be said to belong to the set

I am not sure ; if -1 is in the set then either it is t ot t+2. if it is t, then the other number is t+2 ie 1. If -1 is t+2 then, t is -3. So according to me answer should be C.
_________________

"B"....ques says for a number t , t+2 shd be there we know -1 is there so
-1+2 = 1 MUST be there. I don't think we can extrapolate this and make this an infinite series. Surya do u have the OA ?

"B"....ques says for a number t , t+2 shd be there we know -1 is there so -1+2 = 1 MUST be there. I don't think we can extrapolate this and make this an infinite series. Surya do u have the OA ?

Hey Baner, but we already have this as an infinite series in one direction (positive). But I agree with you - this could be an infinite series in one direction, starting with -1. So we cannot assume the presence of numbers lower than -1.
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"B"....ques says for a number t , t+2 shd be there we know -1 is there so -1+2 = 1 MUST be there. I don't think we can extrapolate this and make this an infinite series. Surya do u have the OA ?

if t+2 exists in the set then it would be t for another number......So it should be valid for all the numbers given.

A number's existence is enough to qualify itself to become t

I agree this is another of those controversial ques, u can argue it both ways....let's wait for the OA...I bet OA is not "E" Surya can u post OA plz and OE if available.

I am not sure ; if -1 is in the set then either it is t ot t+2. if it is t, then the other number is t+2 ie 1. If -1 is t+2 then, t is -3. So according to me answer should be C.

you're right, but the stem doesn't ask for numbers that COULD be in the set (because I agree with you that -3 COULD be in the set, but we're not sure of that, so we can't say that it MUST be there)

"B"....ques says for a number t , t+2 shd be there we know -1 is there so -1+2 = 1 MUST be there. I don't think we can extrapolate this and make this an infinite series. Surya do u have the OA ?

Question says if t is there, then t+2 must be there. That's an infinite series, if ever there was one. However, we can only be sure of this series in one direction

The OA I have got is D. I am not convinced though. Why it has to go in postive direction only. For me it should be C. can any one justify D as answer convincingly.
S
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A set of numbers has the property that for any number t in the set, t + 2 is in the set. If â€“1 is in the set, which of the following must also be in the set? I. -3 II. 1 III. 5

If -1 is in the set, then -1+2=1 is in the set, then 1+2=3 is in the set, then 3+2=5 is in the set. We don't know about -3, however, since we don't have t-2 in the set if t in the set.

The answer is D.
_________________

Keep on asking, and it will be given you;
keep on seeking, and you will find;
keep on knocking, and it will be opened to you.

Re: A set of numbers has the property that for any number t in [#permalink]

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04 Aug 2012, 03:28

Hi, Can someone please clarify my doubts..

Question is 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?

Doubt 1 - Lets say t= -1 ; then t+2 is is set = i.e. 1 is in set.

Since question stem is Must be True, how do we make sure that this pattern will continue. i.e if 1 is in set, then 3 will be in set and so on.. Also

Doubt 2 - Cant we assume that t+2 = -1 then t= -3(should also be in set)

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