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mstaub1
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Post subject: If w+x<0, is w-y>0?  Posted: Tue May 12, 2009 2:45 pm |
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If w+x<0, is w-y>0?
(1) x+y<0
(2) y<x<w
The rephrased question is: w>y?
I got D for this answer but MBA.com says that the answer is B. I realized later that the answers I got for each statement contradict each other which I know cannot be possible. Can someone please help me resolve this?
Statement 2 is clearly sufficient but with statement 1, I used a system of equations and also found the statement to be sufficient. Can someone please explain to me why this doesn't work?
w+x<0
-[x+y<0]
w-y<0
w<y
Thanks
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leovir.22
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Post subject: Re: If w+x<0, is w-y>0? Posted: Wed May 13, 2009 4:25 pm |
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If w+x<0, is w-y>0?
(1) x+y<0
(2) y<x<w
Hi mstaub1,
St 1) x+y<0 Now x can be neg or positive (-3 -7<0 3-7<0). Same goes for y too
Thus we don't know what x and y in terms of sign as well as less than or greater than.
Given to us is w + x< 0 if w = -10 and x = -3 and y = -4. so here w<y but then i can interchange the digits and make w>y also, since i can pick up any digit as long as inequality is satisfied. Hence the statement is insufficient.
Now this problem gets solved as soon as St 2) mentions x lies between y and z. Thus the ans is B
Hope this helps!! :)
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JonathanSchneider
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Post subject: Re: If w+x<0, is w-y>0? Posted: Wed May 13, 2009 5:27 pm |
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| ManhattanGMAT Staff |
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Posts: 480
Location: Durham, NC
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mstaub1, be careful: you cannot subtract one inequality from another like this. You CAN add them, but you can't subtract them. Consider:
5 < 6
-5 < -3
If we were to subtract the bottom inequality from the top one, we would get:
10 < 9
This is clearly not true.
Of course, we COULD add the inequalities, which would yield:
0 < 3
This is true.
Seems you fell into a trap. Careful with those inequalities - they're dangerous!
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ayshaw.asif
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Post subject: Re: If w+x<0, is w-y>0? Posted: Thu Jul 30, 2009 12:12 am |
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Is there a way of solving statement 1 without using numbers. Under pressure I sometimes dont take all the possibilites into consideration and which causes wrong answers.
Instructor can you please help me out thanks!!!
Ayshaw
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mangipudi
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Post subject: Re: If w+x<0, is w-y>0? Posted: Thu Jul 30, 2009 12:56 am |
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If w+x<0, is w-y>0?
(1) x+y<0
adding w to (1) w+x+y < w
or w+x < w-y ( This is all you can get from 1)
w-y > w+x < 0 , there is no way you can conclude that w-y >0 from this.
So insufficient.
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RonPurewal
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Post subject: Re: If w+x<0, is w-y>0? Posted: Fri Aug 07, 2009 7:46 am |
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| ManhattanGMAT Staff |
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ayshaw.asif wrote: Is there a way of solving statement 1 without using numbers. Under pressure I sometimes dont take all the possibilites into consideration and which causes wrong answers.
Instructor can you please help me out thanks!!!
Ayshaw not really. you can try to add the inequalities in statement (1) - you can't subtract them - and you'll notice that nothing much happens (you get w + 2x + y < 0). most of the time, if nothing productive happens (as is the case here), then you can safely bet on "insufficient". but, in such cases, unfortunately, you'll have to test numbers if you want to be 100% sure.
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sagarkhale
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Post subject: Re: If w+x<0, is w-y>0? Posted: Fri Aug 21, 2009 1:06 am |
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hi RonPurewal,
I am unable to understand why subtract operation is not possible on aforementioned two equations.
do you mean we can not use subtraction on two inequalities?
W+X< 0--I
X+Y < 0 --II
I-II
W - Y < 0 which answers the question .
Thank you.
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Kweku.Amoako
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Post subject: Re: If w+x<0, is w-y>0? Posted: Mon Sep 07, 2009 8:12 pm |
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I rephrased the question:
w+x < 0
0 < w-y
-----------
w+ x < w-y ---> x < -y or x+y < 0
so the rephrased question becomes is x+y < 0
1) Clearly solves this problem . Sufficient
2) is an obvious one . Sufficient
Why is this method wrong?
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hisabness
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Post subject: Re: If w+x<0, is w-y>0? Posted: Thu Sep 10, 2009 8:01 pm |
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Kweku.Amoako wrote: I rephrased the question:
w+x < 0
0 < w-y
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w+ x < w-y ---> x < -y or x+y < 0
so the rephrased question becomes is x+y < 0
1) Clearly solves this problem . Sufficient
2) is an obvious one . Sufficient
Why is this method wrong? You have your symbols wrong...If you're going to attempt to rephrase as you have, it should be: W+X < 0 is Y-W<0?
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Kweku.Amoako
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Post subject: Re: If w+x<0, is w-y>0? Posted: Thu Sep 10, 2009 10:30 pm |
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I don't get how my signs are wrong
I only just rewrote the inequality w-y > 0 as 0 < w-y. These two are obviously the same thing.
then since I can add inequalities with the same sign I added to w+x < 0
w+x < 0
+ 0 < w-y
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=w+x < w-y
= x< -y or x+y <0 ...still don't get why this is wrong
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hisabness
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Post subject: Re: If w+x<0, is w-y>0? Posted: Thu Sep 10, 2009 10:50 pm |
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sorry misspoke,
you can't use W-Y as you have since you're trying to determine whether it is greater than 0...it's the unknown you're trying to determine.
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Kweku.Amoako
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Post subject: Re: If w+x<0, is w-y>0? Posted: Fri Sep 11, 2009 12:26 pm |
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still don't get it?
the point of rephrasing is to tranform a complex question into something simple by manipulating the information given. eg what is % of 34 can be rephrased to what is x.
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hisabness
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Post subject: Re: If w+x<0, is w-y>0? Posted: Fri Sep 11, 2009 12:43 pm |
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Kweku.Amoako wrote: still don't get it?
the point of rephrasing is to tranform a complex question into something simple by manipulating the information given. eg what is % of 34 can be rephrased to what is x. right, but you can't use the question you're trying to solve as an input since you don't know if it's factual... 5 + X = 10, doest X +4 = 8? X+4= 8 5+X = 10 2x + 9 = 18 2x = 9 X = 4.5 but we know X equals 5 from the equation, it doesn't also equal 4.5. Not sure how else to explain it.
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Nishant.Chandra
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Post subject: Re: If w+x<0, is w-y>0? Posted: Fri Sep 18, 2009 8:25 am |
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I agree with Kweku. Even I get D.
Well B is sufficient (obvious).
Well A is also sufficient because:
Question stem tells us (1) that w+x<0. (2) Asks us is w-y>0 (Yor N)
In part (2) if we multiply both sides by -1 we get (2A) -w+y<0
Now if we add (1) and 2A: (because the signs are facing the same direction)
w+x<0
-w+y<0
_________
x+y<0
In other words, w-y>0 holds true (given w+x<0 ) as long as x+y<0. Rephrase: (2) hold true given (1) as long as the following equation is satisifed x+y<0.
Now Statement 1 gives us this equation.
Therefore, it should be D.
Can any instructor please help as to why answer is B only.
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nitin_prakash_khanna
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Post subject: Re: If w+x<0, is w-y>0? Posted: Sat Sep 19, 2009 4:00 am |
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let me provide some brief inputs.
It is already mentioned in the thread that mistake you are doing is to use the actual question is your answer.
The question is asking is w-y>0
And After reading the the St.1 , things you know are
w+x <0 from Question Stem &
x+y <0 from St.1
Nothing more is given, dont use the inequality used in the Question.
So the above staments can be true
w=1, y=2 and x= -10
w+x = -9<0
w+y = -8 <0
or
w=2, y=1, x=-10
w+x = -8<0
w+y = -9<0
And as you can see we cant say whether w>y or w<y and hence insufficient.
Somewhere in the thread , the question is raised that why subtraction is not possible for two inequalities.
if you look at subtraction ,
its actually A-B which is equivalent to A + (-1 * B)
what you do is multiply an inequality by -1 and add it to the other but as soon as you multiply the sign flips and it doesnt allow you to add. So addition is allowed , subtraction is not.
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