DS doubt

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DS doubt

by ketkoag » Tue Apr 21, 2009 11:33 am
K is a set of integers such that if the integer r is in K, then r + 1 is also in K. Is 100 in K?
(1) 50 is in K.
(2) 150 is in K.

here could u please tell me that if the statement 2 would be "1000 is in k" then is it enough to prove that 100 is in k?? i mean if it is true then if whenever any integer is in k, 100 will be there in k. right?
please explain this to me!!
Source: — Data Sufficiency |

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by DanaJ » Tue Apr 21, 2009 11:48 am
Actually, it doesn't really work like that.

1. since 50 is in the set, so will 51. If 51 is in the set, so will 52 and so on until you reach 100. So yes, 1 is sufficient to answer the question.

2. is insufficient. We know that 150 is in the set, so every integer FROM THEN ON will also be in the set. However, we can't say if 100 is also in the set, since that r and r + 1 doesn't work "backwards", it only works "forwards". For all we know, the set might start at 150 and continue from then on. However, it might also start from say 25 and lead up to 100 and then "pass" that mark.

My answer is A.

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by anshulseth » Tue Apr 21, 2009 9:45 pm
I think D is the answer.

I dont agree with DanaJ.
For Stmt II- If 150 is in K, we can work both forward and backward.
So, 150=r+1, or 149 is in K, and thus moving backward we reach 100.

Similarly, for Stmt I- If 50 is in K, then 51 is in K, and so on till 100.
But we can work backward too, if 50 is in K, 49 is in K too.

Where does the Q stem suggest this.
"since that r and r + 1 doesn't work "backwards", it only works "forwards". "
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by ketkoag » Tue Apr 21, 2009 11:54 pm
danaj OA is D when first i attempted this question i also marked answer A, but its not the OA,
anshulseth, could u please elaborate more on my doubt that if the statement 2 would be "1000 is in k" then is it enough to prove that 100 is in k?? i mean if it is true then if whenever any integer is in k, 100 will be there in k. right? coz u said that it wors forward as well backward.coz i personally feel that it should be forward only as in the question it is mentioned that if the integer r is in K, then r + 1 is also in K .
experts please help..

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by Feep » Tue Apr 21, 2009 11:59 pm
No, the answer to the question as stated is A. The only way it could be D is if the question stated if AND ONLY IF the integer r is in K, then r+1 is in K.

Something is wrong with the question or answer as presented in this thread.
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by DanaJ » Wed Apr 22, 2009 12:10 am
if the integer r is in K, then r + 1 is also in K

This tells me that:

"r is in K" => "r + 1 is in K"

If it were smth like "if the integer r is in K, then r + 1 is also in K and vice versa", then it would be more like:

"r is in K" <=> "r + 1 is in K"

What is the source of this question? Not all sources are 100% reliable.

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by Ian Stewart » Wed Apr 22, 2009 2:18 am
Dana and Feep are absolutely right - the answer is certainly A, not D. Where are the question and OA from?
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by typhoonguywlblwu » Wed Apr 22, 2009 10:19 am
IMO the answer certainly is d.

Statement 1 is correct.If 50 is in k then 51 be in k and so on ....
Therefore, (1) is sufficient

Statement 2 .The set definition is 'K is a set of integers such that if the integer r is in K, then r + 1 is also in K.' Nowehere it states that the beginning with a certain element,the definition for set is valid.
Hence,If 150 is in K,then 149 will be in k and so on...
Statement (2) is sufficient as well.


If we have an assumption that the set could begin at 150,hence (2) is not valid,then theres nothing stopping us to assume that set set could only have elements till 99,in which case (1) becomes insufficient as well.


The OA D is correct IMHO.

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by Ian Stewart » Thu Apr 23, 2009 2:34 pm
typhoonguywlblwu wrote:IMO the answer certainly is d.

Statement 1 is correct.If 50 is in k then 51 be in k and so on ....
Therefore, (1) is sufficient

Statement 2 .The set definition is 'K is a set of integers such that if the integer r is in K, then r + 1 is also in K.' Nowehere it states that the beginning with a certain element,the definition for set is valid.
Hence,If 150 is in K,then 149 will be in k and so on...
Statement (2) is sufficient as well.


If we have an assumption that the set could begin at 150,hence (2) is not valid,then theres nothing stopping us to assume that set set could only have elements till 99,in which case (1) becomes insufficient as well.


The OA D is correct IMHO.
No, Statement 2 is not sufficient.

When considering Statement 1, you cannot, as you suggest, have a set which 'could only have elements till 99', because if 99 is in the set, we know from the stem that 99+1 = 100 must also be in the set (since we know that 'if the integer r is in K, then r + 1 is also in K').

When we know, from Statement 2, that 150 is in the set, we can be certain that 151 is in the set, since "if the integer r is in K, then r + 1 is also in K." And since 151 is in the set, we can be sure 152 is in the set, and so on. But we can't be certain that 149 is in the set; our set could be the infinite set:

{150, 151, 152, 153, 154, ...}

This set fits with the information in Statement 2, and also with the information provided in the stem; whenever r is in the set, r+1 is in the set.
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by Stuart@KaplanGMAT » Thu Apr 23, 2009 2:53 pm
Let's turn this into a non-math question to illustrate why (2) is NOT sufficient. Let's assume that the following statement is true:

"If Bob is dead today, he will still be dead tomorrow."

Now let's look at a question and statements analogous to those in the original post:

Q: Was Bob dead on July 15th?

(1) Bob was dead on July 1st.

(2) Bob was dead on September 3rd.

From (1), we know that by July 1st, Bob was dead. The rule tells us that Bob stays dead, so we know for sure that he'll still be dead on July 15th: sufficient.

From (2), we know that by September 3rd, Bob was dead. So, we know that Bob will be dead every day after Septmeber 3rd. Can we infer from this exactly when Bob died? Of course not - he could have died on September 3rd itself, or the day before, or a month before, or 100 years before - there's absolutely no way for us to tell. So, was he dead on July 15th? Possibly, but not for sure: insufficient.

(My apologies to anyone named Bob reading this post!)

(And there must be something in the coding that doesn't like the word "dead" - no clue why "@" is showing up instead of "a"!)
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by Ian Stewart » Thu Apr 23, 2009 4:20 pm
Brilliant stuff, Stuart!
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by cramya » Thu Apr 23, 2009 5:09 pm
Both u guys are awesome!

Stuart,
That's one fantastic "Bob" example.... :lol:


The correct OA for this is A and not D as explained by Ian/Stuart and others in their posts.

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by ketkoag » Fri Apr 24, 2009 3:59 am
thanks a lot guys for the efforts.. u've given the excellent examples that clear all the doubts regarding the question.. i am sure GMAT will take care of such an ambiguity presented in this ques.. :)
thanks again..

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by Musiq » Fri Apr 24, 2009 10:41 am
Going from Stuarts example; this question is really more Critical Reasoning than it is Data Sufficiency.

The Classic Causaility trap is to assume that if X CAUSES Y, then Y MUST CAUSE X.

This is not true....."When it eat a cookie I drink milk ......doesnt NECESSARILY mean that when I drink milk I eat a cookie"

So, If r is present in Set K; then r+ 1 MUST be in SET K (this is a fact given to us in the question.

But we cannot say with ABSOLUTE CONVICTION that if r+ 1 is present in the SET K, then r MUST be present.

This "MAYBE-NESS" translates to Insuffcienecy is Yes/NO DS.

Answer is A

Also, some of us are of the opinion that GMAT is ambigous on this question. I do not know the source of this question, but I can guarantee that this question is no more ambigous than a 650+ Data Sufficiency question should be.
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