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18 Mar 2015, 20:31
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C
Be sure to select an answer first to save it in the Error Log before revealing the correct answer (OA)!
Difficulty:
65%
(hard)
Question Stats:
54% (01:48) correct
46%
(01:44)
wrong
based on 843
sessions
History
Date
Time
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Not Attempted Yet
ColdSushi
Is z>0?
(1) (Z+1)(Z)(Z-1) < 0
(2) |Z|< 1
(1) (Z+1)(Z)(Z-1) < 0
(2) |Z|< 1
Is Z > 0
(1) (Z+1)(Z)(Z-1) < 0
There are 3 transition points -1, 0 and 1. The expression will be negative if Z < -1 or 0 < Z < 1.
So Z may be positive or negative in this range. Not sufficient.
(2) |Z|< 1
-1 < Z < 1
Z may be positive or negative in this range. Not sufficient.
Which range of Z satisfies both inequalities? 0 < Z < 1
In this range, Z is always positive. Sufficient.
Answer (C)
18 Mar 2015, 23:20
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Hi ColdSushi,
This DS question is loaded with Number Property rules; you'll actually see Number Properties on many DS questions, so it's a good area to spend some extra time on.
We're asked if X > 0. This is a YES/NO question.
Fact 1: (Z+1)(Z)(Z-1) < 0
This looks like a great opportunity to TEST VALUES. Before randomly TESTing a value though, I want to note the Number Property patterns in this question. Since the product is LESS than 0, we cannot have a product that equals 0.
Looking at the three parentheses, we can see that the product will equal 0 if Z is -1, 0 or 1. This means that we CANNOT TEST any of those values.
The first TEST should be something fairly obvious. We need a negative product, so let's choose a negative value.
IF...
Z = -2
(-1)(-2)(-3) = -6, which is < 0
The answer to the question is NO.
Notice how in the first Test we had the product of 3 negative numbers. We can also get a negative product with just 1 negative number, so we should look for that option next.
IF....
Z = 1/2
(3/2)(1/2)(-1/2) = -3/8, which is < 0
The answer to the question is YES.
Fact 1 is INSUFFICIENT
Fact 2: |Z| < 1
This means that -1 < Z < 1
IF...
Z = 1/2
The answer to the question is YES
IF...
Z = -1/2
The answer to the question is NO
Fact 2 is INSUFFICIENT
Combined, we know....
(Z+1)(Z)(Z-1) < 0
-1 < Z < 1
Since Z is greater than -1, there's no way for (Z+1) to be negative. By extension, we can't end up with the product of 3 negative numbers, so the only way to satisfy the first Fact is if we have JUST 1 negative number (which will be the (Z-1) piece of the product. Z itself must be positive....which will give us...
(Z+1)(Z)(Z-1)
(+)(+)(-)
The answer to the question is ALWAYS YES
Combined, SUFFICIENT
Final Answer:
GMAT assassins aren't born, they're made,
Rich
This DS question is loaded with Number Property rules; you'll actually see Number Properties on many DS questions, so it's a good area to spend some extra time on.
We're asked if X > 0. This is a YES/NO question.
Fact 1: (Z+1)(Z)(Z-1) < 0
This looks like a great opportunity to TEST VALUES. Before randomly TESTing a value though, I want to note the Number Property patterns in this question. Since the product is LESS than 0, we cannot have a product that equals 0.
Looking at the three parentheses, we can see that the product will equal 0 if Z is -1, 0 or 1. This means that we CANNOT TEST any of those values.
The first TEST should be something fairly obvious. We need a negative product, so let's choose a negative value.
IF...
Z = -2
(-1)(-2)(-3) = -6, which is < 0
The answer to the question is NO.
Notice how in the first Test we had the product of 3 negative numbers. We can also get a negative product with just 1 negative number, so we should look for that option next.
IF....
Z = 1/2
(3/2)(1/2)(-1/2) = -3/8, which is < 0
The answer to the question is YES.
Fact 1 is INSUFFICIENT
Fact 2: |Z| < 1
This means that -1 < Z < 1
IF...
Z = 1/2
The answer to the question is YES
IF...
Z = -1/2
The answer to the question is NO
Fact 2 is INSUFFICIENT
Combined, we know....
(Z+1)(Z)(Z-1) < 0
-1 < Z < 1
Since Z is greater than -1, there's no way for (Z+1) to be negative. By extension, we can't end up with the product of 3 negative numbers, so the only way to satisfy the first Fact is if we have JUST 1 negative number (which will be the (Z-1) piece of the product. Z itself must be positive....which will give us...
(Z+1)(Z)(Z-1)
(+)(+)(-)
The answer to the question is ALWAYS YES
Combined, SUFFICIENT
Final Answer:
GMAT assassins aren't born, they're made,
Rich
