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DS questions that stumped me from GMATPrep2

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DS questions that stumped me from GMATPrep2 [#permalink] New post 07 Dec 2011, 10:43
00:00
A
B
C
D
E

Difficulty:

  5% (low)

Question Stats:

56% (01:15) correct 44% (01:28) wrong based on 9 sessions
I took my first GmatPrep today after studying Quant for a month(Working on a 3month plan suggested by gmatclub experts). I haven't touched Verbal yet and my score was 660 (Q49V31) although i was a little disturbed about my Verbal score since i expected better, i was pretty surprised how getting 13 questions wrong in Quant got me to 49. But since GMAT is adaptive i guessed its possible.
Anyways, i reworked the incorrect questions after the exam and cracked a few of them, however there are a few others that just stumped me completely even after giving them a 2nd shot.

1. If ab!=0 and point (-a,b) and (-b,a) are in the same quadrant ,does point (-x,y) lie in this quadrant?
i) xy>0
ii) ax>0

There are a few others coming up..Please let me know if I made a rookie mistake :beat by posting these here when it should be in some other forum category, I searched a lot couldn't really find any other suitable place. Thanks :thanks
[Reveal] Spoiler: OA

Last edited by ijoshi on 07 Dec 2011, 17:28, edited 2 times in total.
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Re: DS questions that stumped me from GMATPrep2 [#permalink] New post 07 Dec 2011, 14:28
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ijoshi wrote:
i was pretty surprised how getting 13 questions wrong in Quant got me to 49. But since GMAT is adaptive i guessed its possible.


I just wrote a post about this exact question on another forum, so I'll just paste a link if you're interested:

http://www.beatthegmat.com/gmat-scoring ... 98484.html


ijoshi wrote:
1. If ab!=0 and point (-a,b) and (-b,a) are in the same quadrant ?
i) xy>0
ii) ax>0


I think you've missed part of the question here - I think the question says:

If (-a, b) and (-b, a) are in the same quadrant, is the point (-x, y) in the same quadrant as (-a, b)?
1) xy > 0
2) ax > 0


If two points are in the same quadrant, then their x-coordinates have the same sign, and their y-coordinates have the same sign. So from the information that (-a, b) and (-b, a) are in the same quadrant, we learn that a and b have the same sign (either by looking at x-coordinates or at y-coordinates). So we know that a and b are either both positive or both negative, and that the point (-a, b) thus has one negative coordinate and one positive coordinate. The problem is we don't know which coordinate is positive, and which negative; it could be (+, -) or it could be (-, +).

From Statement 1, we learn that x and y have the same sign. Thus the point (-x, y) has coordinates of opposite signs. This point could be (+, -) or (-, +), so we don't know if it's in the same quadrant as (-a, b).

Statement 2 doesn't mention y at all, so cannot be sufficient, since we need to know about the sign of y.

Combining the two statements we know a and b have the same sign (from the stem), a and x have the same sign (from Statement 2) and x and y have the same sign (from Statement 1). So a, b, x and y all have the same sign. Thus the x-coordinates of (-a, b) and (-x, y) have the same sign, as do their y-coordinates, and the two points must be in the same quadrant. The answer is C.
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Re: DS questions that stumped me from GMATPrep2 [#permalink] New post 07 Dec 2011, 17:38
IanStewart wrote:
ijoshi wrote:
i was pretty surprised how getting 13 questions wrong in Quant got me to 49. But since GMAT is adaptive i guessed its possible.


I just wrote a post about this exact question on another forum, so I'll just paste a link if you're interested:



Great post!
and thank u for the reply.Had in fact missed a part of the question(kind of a major one),edited it.
:-D thanks
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Re: DS questions that stumped me from GMATPrep2 [#permalink] New post 18 Dec 2011, 15:48
(-a, b) and (-b, a) are in the same quadrant.
Is the point (-x, y) in the same quadrant as point (-a, b)?

(1) xy > 0
(2) ax > 0

From the information that (-a, b) and (-b, a) are in the same quadrant, it can be determined that
(-a, b) is in either quadrant II or quadrant IV. If (x, y) and (a, b) are in the same exact quadrant, they will have the same sign and (-x, y) will be in the same quadrant as (-a, b)'s.

(1) xy > 0

(x, y) is in quadrant I or quadrant III.
(-x, y) is in quadrant II or quadrant IV.
No further information is provided about (-a, b).

(2) ax > 0

Point x in (x, y) has the same sign as does point a in (a, b). Since a and b have the same sign, x, a and b have the same sign.

But the sign of point x could be different from, or the same as, the sign of point y. The condition that (x, y) and (a, b) have the same sign, and therefore that (-x, y) and (-a, b) are in the same quadrant, is possible but uncertain.

Combined analysis:

x has the same sign as y
x has the same sign as a and b
x, y, a and b all have the same sign.

This means (x, y) and (a, b) are in the same quadrant. (-x, y) and (-a, b) are in the same quadrant.


[xyab+xdj]

Last edited by Study1 on 31 Jan 2012, 15:26, edited 3 times in total.
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Re: DS questions that stumped me from GMATPrep2 [#permalink] New post 18 Dec 2011, 17:23
@IanStewart - Nice explanation. kudos to you!
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Re: DS questions that stumped me from GMATPrep2   [#permalink] 18 Dec 2011, 17:23
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