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Is A/B < 0 - Help

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Is A/B < 0 - Help [#permalink]  17 Jul 2009, 16:10
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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

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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Re: Is A/B < 0 - Help [#permalink]  17 Jul 2009, 16:57
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.
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Last edited by vannu on 17 Jul 2009, 17:03, edited 1 time in total.
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Re: Is A/B < 0 - Help [#permalink]  17 Jul 2009, 17:03
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..
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Re: Is A/B < 0 - Help [#permalink]  17 Jul 2009, 20:46
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.
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Re: Is A/B < 0 - Help [#permalink]  17 Jul 2009, 22:26
[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

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Re: Is A/B < 0 - Help [#permalink]  17 Jul 2009, 23:47
Wengen wrote:
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 ?

You can rewrite the question this way only if you are sure that b>0, but it is not given. It can be either b<0 and a>0 or b>0 and a<0.
Statement 1 gives us b>0, nothing about a - insuff.
Statement 2 gives us a<0, nothing about b - insuff.
Together - sufficient, C.
Re: Is A/B < 0 - Help   [#permalink] 17 Jul 2009, 23:47
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