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is d negative

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rachelhong2012
 Post subject: is d negative
 Post Posted: Sat Nov 26, 2011 2:48 pm 
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Course Students


Posts: 47
Hi,

I wonder if my approach to this problem is right:

Is d negative?

(1) e + d = -12

(2) e – d < -12



Since either statement alone is sufficient and we must combine them. I wonder if I can add the two statements together and first cancel out d to get:

2e < -24 so e<-12
Then plug that value back into either statement and get: d is a positive number?

Thanks
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tim
 Post subject: Re: is d negative
 Post Posted: Tue Dec 13, 2011 2:46 am 
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ManhattanGMAT Staff


Posts: 2242
Location: Southwest Airlines, seat 21C
With your approach you can’t just plug e<-12 into either statement, because plugging into statement 2 is inconclusive. However, since you have both statements to use, you still have sufficient information because plugging into 1 tells you d is positive. A better approach though would be to subtract statement 1 from statement 2, getting rid of the e’s you don’t want and keeping the d’s you want:

-2d < 0
d > 0

_________________
Tim Sanders
Manhattan GMAT Instructor
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rachelhong2012
 Post subject: Re: is d negative
 Post Posted: Fri Dec 23, 2011 4:44 pm 
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Course Students


Posts: 47
tim wrote:
With your approach you can’t just plug e<-12 into either statement, because plugging into statement 2 is inconclusive. However, since you have both statements to use, you still have sufficient information because plugging into 1 tells you d is positive. A better approach though would be to subtract statement 1 from statement 2, getting rid of the e’s you don’t want and keeping the d’s you want:

-2d < 0
d > 0


Thanks Tim.

I wonder how did you "subtract" inequality?

I know you can multiple by -1 as a way to "subtract", but you have to switch the sign when you do that.

So I will multiple (e-d<-12) by -1 and get -e + d > 12
combine/add the rephrased inequality with e + d = -12
I will cancel out e and get 2d > 0 hence d>0, am I right?

Thanks again.
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tim
 Post subject: Re: is d negative
 Post Posted: Tue Dec 27, 2011 1:23 pm 
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ManhattanGMAT Staff


Posts: 2242
Location: Southwest Airlines, seat 21C
As a reminder, you should NEVER subtract inequalities. Please take a close look at my post, and you will notice that i did not subtract an inequality.. :)

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Tim Sanders
Manhattan GMAT Instructor
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shubha2110
 Post subject: Re: is d negative
 Post Posted: Wed Dec 28, 2011 10:16 am 
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Students


Posts: 5
Hi Tim,

Could you please explain further how did you get -2d < 0? I am confused how you got it without subtracting + you say we can't subtract inequalities (why?)

In this question, I found that the 2nd statement tells that d > e.

I solved it as below:

e - d < -12

so, multiplying by -1 on both sides,

d - e > 12

For this to be true, d > e

Then, statement 1 says that e + d = -12. Can i use the fact that d > e in statement 1 to solve conceptually? I think I am missing something.

Hope you can help.
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tim
 Post subject: Re: is d negative
 Post Posted: Sat Dec 31, 2011 1:43 pm 
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ManhattanGMAT Staff


Posts: 2242
Location: Southwest Airlines, seat 21C
i DID subtract; i just didn’t subtract INEQUALITIES. Part of success on the GMAT is being able to identify these subtle differences, so please pay careful attention.. :)

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Tim Sanders
Manhattan GMAT Instructor
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