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# Is A/B < 0 - Help

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Intern
Joined: 20 Jun 2009
Posts: 7
Is A/B < 0 - Help [#permalink]

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17 Jul 2009, 17:10
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Hi Guys,
Hello All,

Came across this question in mgmat practice

Is a/b 0
2) ab^4<0

Since the original question is - Is a/b<0, can I rewrite the question as Is a<0 ? If I do this, I arrive at (2) is enough to answer. But as per MGMAT, (1) and (2) are sufficient to answer. What am I missing? Why should'nt I rewrite the q as is a<0 ?

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Manager
Joined: 16 Apr 2009
Posts: 213
Schools: Ross
Re: Is A/B < 0 - Help [#permalink]

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Updated on: 17 Jul 2009, 18:03
Quote:

Is a/b < 0

1) a^2/b^3 > 0
2) ab^4<0

a/b < 0

Let a be -2 and b be 3

stat 1
a^2/b^3 > 0
=(-2)^2/ 3^3 =4/27
Here we are not sure whether a is 2 or -2

insuff
a can be -ve or +ve

stat 2
ab^4<0
(-2)(3)^4
-2*81 will give a negative value

This clearly tells that a is negative
hence suff

OA is B

I am not sure why the answer is D. Let's wait for the math gurus to come up with an explanation.
_________________

Keep trying no matter how hard it seems, it will get easier.

Originally posted by vannu on 17 Jul 2009, 17:57.
Last edited by vannu on 17 Jul 2009, 18:03, edited 1 time in total.
Intern
Joined: 20 Jun 2009
Posts: 7
Re: Is A/B < 0 - Help [#permalink]

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17 Jul 2009, 18:03
Thanks vannu. My question is why can't a/b>0, rephrased as is a>0? If we do that, we'll get a different result right...I'm sorry I'm missing something very basic..
Senior Manager
Joined: 20 Mar 2008
Posts: 439
Re: Is A/B < 0 - Help [#permalink]

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17 Jul 2009, 21:46
a/b>0 cannot be written as a>0 as both a and b can be negative. In that case a>0 st. won't hold true.
For example, let a= -1; b=-2. Then a/b = -1/-2 = 1/2 >0.
But if we say, a>0 that won't be true in all cases.

Now with st.1 says: 1) a^2/b^3 > 0
With that we know, b cannot be negative for a^2/b^3 to be > 0. But a can be either.

Question asked is: is a/b < 0.

Since we know that b is positive (from st.2,) we can deduce that a must be negative for a/b to be less than 0.

If a is +ve, then a/b becomes +ve or a/b > 0

Seems to me OA --> D is right.
Retired Moderator
Joined: 05 Jul 2006
Posts: 1734
Re: Is A/B < 0 - Help [#permalink]

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17 Jul 2009, 23:26
[quote="Wengen"]Hi Guys,
Hello All,

Came across this question in mgmat practice

Is a/b < 0

1) a^2/b^3 > 0
2) ab^4<0

Manager
Joined: 15 Apr 2008
Posts: 50
Location: Moscow
Re: Is A/B < 0 - Help [#permalink]

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18 Jul 2009, 00:47
[quote="Wengen"]
Since the original question is - Is a/b0, but it is not given. It can be either b0 or b>0 and a0, nothing about a - insuff.
Statement 2 gives us a<0, nothing about b - insuff.
Together - sufficient, C.

--== Message from GMAT Club Team ==--

This is not a quality discussion. It has been retired.

If you would like to discuss this question please re-post it in the respective forum. Thank you!

To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.
Re: Is A/B < 0 - Help   [#permalink] 18 Jul 2009, 00:47
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