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Gmat prep not happy with OA !

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Junior | Next Rank: 30 Posts
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by mazwaz » Sun Apr 10, 2011 8:10 am
is m + z > 0 ?

(1) m - 3z > 0
(2) 4z - m > 0

Clearly each statement alone is not sufficient!!!
and together we get : 3z < m < 4z
so if z = -2 then m + z < 0
if z = 2 then m + z > 0

I don't understand how it can't be OA E ???
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Source: — Data Sufficiency |

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by Brent@GMATPrepNow » Sun Apr 10, 2011 8:23 am
mazwaz wrote:is m + z > 0 ?

(1) m - 3z > 0
(2) 4z - m > 0

Clearly each statement alone is not sufficient!!!
and together we get : 3z < m < 4z
so if z = -2 then m + z < 0
if z = 2 then m + z > 0

I don't understand how it can't be OA E ???
This is a tricky question.
Let's start with your (very correct) conclusion that 3z < m < 4z
Notice that we can use this to conclude that 3z < 4z
From here, if we subtract 3z from both sides, we get 0 < z
In other words, z is positive.
If z is positive, then 3z is positive, and since 3z < m, we know that m is positive.

If z and m are both positive, then m+x > 0, so the answer is C.

Aside: notice that z cannot equal -2 (as you suggested) since that would contradict the fact that 3z < 4z

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
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Junior | Next Rank: 30 Posts
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by mazwaz » Sun Apr 10, 2011 8:27 am
ohh yeah I didn't see that!!
Now it is clear!
Thanks for your quick reply!!!
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by HSPA » Sun Apr 10, 2011 6:52 pm
Add the two statements: m-3z+4z-m >0 => z > 0

Now using m-3z>0 => m > 3Z => m is +ve, z is positive

thus m+z is +ve... IMO C
First take: 640 (50M, 27V) - RC needs 300% improvement
Second take: coming soon..
Regards,
HSPA.
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by force5 » Mon Apr 11, 2011 4:07 am
z>0 gives C
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