Problem Solving

I am not sure about the OA. Pls help.

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

--> E

pepeprepa wrote:--> E

Not the right one.

I thought E to, but the question asks, which MUST be in the set and if that's the case, then the answer is I and II - C.

If -1 is in the set, that could be t or that could be t+2. So if it's t then 1 is in the set, you keep II. if it's the product of t + 2 then -3 is in the set. Since you dont know, then they both must be in the set. 5 COULD be in the set if you extrapolated out, but it doesn't HAVE to be in the set. That's a pretty nasty question (not nasty as in hard, but nasty as in we're trying to trick you)

Thank you for the explanation, the must is very important as you said.
Is this kind of trap common in the gmat?

well, I would go for D.

the elements in the Set are: t, and t+2

so,

-1 +2 =1

1+2 =3

3+2 = 5


What's the OA?

Carol wrote:well, I would go for D.

the elements in the Set are: t, and t+2

so,

-1 +2 =1

1+2 =3

3+2 = 5


What's the OA?

OA is D. Thanks for the explanation.

Mclaughlin wrote:I thought E to, but the question asks, which MUST be in the set and if that's the case, then the answer is I and II - C.

If -1 is in the set, that could be t or that could be t+2. So if it's t then 1 is in the set, you keep II. if it's the product of t + 2 then -3 is in the set. Since you dont know, then they both must be in the set. 5 COULD be in the set if you extrapolated out, but it doesn't HAVE to be in the set. That's a pretty nasty question (not nasty as in hard, but nasty as in we're trying to trick you)

Thanks for this question and above all the explanation. This question will always remind me of one thing on GMAT: ASSUME NOTHING!!!

duke wrote:

Carol wrote:well, I would go for D.

the elements in the Set are: t, and t+2

so,

-1 +2 =1

1+2 =3

3+2 = 5


What's the OA?

OA is D. Thanks for the explanation.

It can't be D. It has to be C. Duke, you said in your orig. post that you didn't know that OA but now you say the OA is D when it isn't. 5 does not HAVE to be in the set, but I and II MUST be in the set. where is this question is from.

From the font it would appear this question is from the GMAT prep software distributed b GMAC.

Here is how I understood the question.

1. We know -1 is in the set.
2. Therefore -1 MUST be t or the result of t+2
3. If -1 is t then 1 MUST be in the set
4. If -1 is the result of t+2 then -3 MUST be in the set.
5. In order for 5 to be in the set we have to assume 3 and/or 5 are in the set. I don't think we can do that.

The answer would be C in this case.

Does that seem right?

Wow.. this really isn't a complicated problem. Shouldn't be beyond the 500-600 range.

The question is saying that for "any number t in the set", t+2 will also be in the set. They aren't saying one specific number is t... rather "pick any number in the set, and let's call it t"

So if we were to learn that the number 5 is in the set, we'd then know that 7 is in the set as well.. And if 7 is in the set, 9 must also be in the set.... etc. etc. The set continues on infinitely.

So the question is, which numbers HAVE to be in this set.

Since we know that -1 is in the set, we know that 1,3,5,7,9,11,.... are also going to be in the set. What we don't know is where the set starts.

It could start at -1342345, or -1 could be the first number (or it could start at -2 and have all integers >=-2 in it). So we can only say, for certain, that II and III are in it. So the answer is D.

Meanwhile, even if the question had be asked as you understood it (as "In a set, one element is t... and the set also contains the number t+2. -1 is an element of the set.") your logic is way off! First of all, there would be no reason to assume that -1 is t or t+2. The set could contain the following numbers {-1,10,12} for example. Second, even if we were told that -1 was either t+2 or t... (perhaps by asking a question like "In a set with two numbers, one number is two greater than the other. -1 is in the set.") We'd have no way of knowing whether -1 is t+2 or t... If we don't know which it is, we CERTAINLY CAN'T say it's both!!!

Mclaughlin wrote:I thought E to, but the question asks, which MUST be in the set and if that's the case, then the answer is I and II - C.

If -1 is in the set, that could be t or that could be t+2. So if it's t then 1 is in the set, you keep II. if it's the product of t + 2 then -3 is in the set. Since you dont know, then they both must be in the set. 5 COULD be in the set if you extrapolated out, but it doesn't HAVE to be in the set. That's a pretty nasty question (not nasty as in hard, but nasty as in we're trying to trick you)

Mclaughlin wrote:

duke wrote:

Carol wrote:well, I would go for D.

the elements in the Set are: t, and t+2

so,

-1 +2 =1

1+2 =3

3+2 = 5


What's the OA?

OA is D. Thanks for the explanation.

It can't be D. It has to be C. Duke, you said in your orig. post that you didn't know that OA but now you say the OA is D when it isn't. 5 does not HAVE to be in the set, but I and II MUST be in the set. where is this question is from.

@Mclaughlin...I didn't say I do not know the OA, but said I'm not sure about the OA. Carol's explanation helped me to understand the question correctly. Initially I chose wrong one (C) and couldn't understand why D is to be the answer. Now, I think I'm clear.

duke wrote:I am not sure about the OA. Pls help.

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

Any 1 canhelp with OA here.

IMO C

Wow.. blast from the past! As I said a year and a half ago, before I scored 99th percentile on the gmat, the answer is unambiguously D. In case the old posts are enough of an explanation here it goes again:

The set is going to be infinitely long and will continue increasing positively. Any number that's in the set will also have the same number + 2 (t+2). So if we know that 4 is in the set, 6, 8, 10,12, 14, 16, 18,20, etc, etc, etc are also definitely there.

So if we know that -1 is in the set, we know with certainty that 1,3,5,7,etc are also in the set.

What we don't know is where the trend starts. -1 could be the lowest number, but it could also be -3, or -5, or even -4.. we just don't know where it starts. Therefore, the only thing we can conclude with certainty is that -1,1,3,5,7,9, etc are in the set. So the answer is, once again, unambiguously, D.