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Each week we'll be posting several questions from The Official Guide For GMAT® Quantitative Review, 2ND Edition and then after couple of days we'll provide Official Answer (OA) to them along with a slution.

We'll be glad if you participate in development of this project: 1. Please provide your solutions to the questions; 2. Please vote for the best solutions by pressing Kudos button; 3. Please vote for the questions themselves by pressing Kudos button; 4. Please share your views on difficulty level of the questions, so that we have most precise evaluation.

Re: K is a set of numbers such that [#permalink]
30 Jan 2014, 22:48

1

This post received KUDOS

Expert's post

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This post was BOOKMARKED

SOLUTION

K is a set of numbers such that

(i) if x is in K, then -x is in K, and (ii) if each of x and y is in K, then xy is in K.

Is 12 in K?

(1) 2 is in K --> according to (i) -2 is n K --> according to (ii) -2*2=-4 is in K --> according to (i) -(-4)=4 is in K and so on. Thus we know that 2, -2, -4, 4, 8, -8, 16, -16, ... are in K, so basically powers of 2 and their negative pairs. Is 12 in K? We don't know. Not sufficient.

(2) 3 is in K --> according to (i) -3 is n K --> according to (ii) -3*3=-9 is in K --> according to (i) -(-9)=9 is in K and so on. Thus we know that 3, -3, -9, 9, 27, -27, 81, -81, ... are in K, so basically powers of 3 and their negative pairs. Is 12 in K? We don't know. Not sufficient.

(1)+(2) From (1) 4 is in K and from (2) 3 is in K, hence according to (ii) 4*3=12 must also be in K. Sufficient.

Re: K is a set of numbers such that [#permalink]
31 Jan 2014, 00:08

Let's analyze statements with what's given in the question.

Statement (1) 2 is in K (i) -2 is also in K (ii) 2 and -2 are both in K, so -4 is also in K .... then +4... and then -8 and +8 are in K ... looks like 2^n and -2^n are included where n is integer which does not include 12 definitely... So Statement (1) is sufficient to answer the question "Is 12 in K?", answer being no.

Statement (2) Similarly results in a set 3^n and -3^n , which again answers our question, that 12 is definitely not part of the set K. So Statement (2 is sufficient to answer the question "Is 12 in K?", answer being no.

Answer D; Both statement 1 & statement 2 are ALONE sufficient.

[oops It turns out my answer was wrong ... Just leaving the post as it is, so you know what not to do ] _________________

Kudos (+1) if you find this post helpful.

Last edited by code19 on 31 Jan 2014, 12:59, edited 4 times in total.

Re: K is a set of numbers such that [#permalink]
31 Jan 2014, 00:59

1

This post received KUDOS

Ans. C From S1:if 2 is in the series,then -2 will also be there. And if 2 & -2 are there -4 will be there.If -4 is in the series, 4 will also be there...and so on The series becomes:2,-2,4,-4...powers of 2 But the stimulus remains silent about what is not there in this series.So insufficient.(12 might or might not be there.)

Same explanation for S2:The series will have numbers with powers of 3.

Together for S1 & S2,at some point we'll have multiple of 3 and 4 because if 3 and 4 are there in the series,their multiple will definitely be there as implied by the second statement in stimulus.Sufficient.

Re: K is a set of numbers such that [#permalink]
01 Feb 2014, 09:30

Expert's post

SOLUTION

K is a set of numbers such that

(i) if x is in K, then -x is in K, and (ii) if each of x and y is in K, then xy is in K.

Is 12 in K?

(1) 2 is in K --> according to (i) -2 is n K --> according to (ii) -2*2=-4 is in K --> according to (i) -(-4)=4 is in K and so on. Thus we know that 2, -2, -4, 4, 8, -8, 16, -16, ... are in K, so basically powers of 2 and their negative pairs. Is 12 in K? We don't know. Not sufficient.

(2) 3 is in K --> according to (i) -3 is n K --> according to (ii) -3*3=-9 is in K --> according to (i) -(-9)=9 is in K and so on. Thus we know that 3, -3, -9, 9, 27, -27, 81, -81, ... are in K, so basically powers of 3 and their negative pairs. Is 12 in K? We don't know. Not sufficient.

(1)+(2) From (1) 4 is in K and from (2) 3 is in K, hence according to (ii) 4*3=12 must also be in K. Sufficient.

Re: K is a set of numbers such that [#permalink]
19 Feb 2014, 06:01

Bunuel, one quick query -> When we say that (from stmt 1) 2 is there in the set and hence -2 is also there -> Here we take 2 and -2 as x and -x, but then we also apply the logic x*y = -4 (here we consider 2 as x and -2 as y (and not as -x)). Could there be a flaw in the problem statement?

Bunuel wrote:

pradeepss wrote:

Bunel can you update the oa? It shows as d on gmat timer.

Re: K is a set of numbers such that [#permalink]
19 Feb 2014, 06:52

Expert's post

sunnymon wrote:

Bunuel, one quick query -> When we say that (from stmt 1) 2 is there in the set and hence -2 is also there -> Here we take 2 and -2 as x and -x, but then we also apply the logic x*y = -4 (here we consider 2 as x and -2 as y (and not as -x)). Could there be a flaw in the problem statement?

Bunuel wrote:

pradeepss wrote:

Bunel can you update the oa? It shows as d on gmat timer.

________________ Done. Thank you.

(i) and (ii) are general rules for the set, meaning that they apply to any numbers in the set:

(i) if a number is in K, then - that number is also in K (ii) for any two numbers in the set, their product is also in the set.

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