Hi james,
This question is perfect for TESTing Values:
We're asked if B < 0. This is a YES/NO question.
Fact 1: B is a multiple of 13.
By definition, a multiple can be positive, negative or 0.
So B could be....
13(1) = 13 Positive results gives us a NO answer
13(2) = 26
etc.
13(0) = 0 "Null" result gives us a NO answer
13(-1) = -13 Negative results give us a YES answer
13(-2) = -26
etc.
Fact 1 is INSUFFICIENT.
Fact 2: B^3 < 13
Here we have no restrictions on "multiples", so any number that fits this restriction will do...
If....
B = 1, then the answer is NO
B = -1, then the answer is YES
Fact 2 is INSUFFICIENT
Combined, we know...
B is a multiple of 13
B^3 < 13
If B = 0, then the answer to the question is NO
If B = -13, then the answer to the question is YES
Combined, INSUFFICIENT
Final Answer: E
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Is b < 0?
Source: Beat The GMAT — Data Sufficiency |
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Target question: Is b negative?james wrote:Is b < 0?
(i) b is a multiple of 13
(ii) b³ < 13
Statement 1: b is a multiple of 13
Here are some multiples of 13: ....-39, -26, -13, 0, 13, 26, 39,...
As you can seem there are several values of b that satisfy the given condition. Let's examine two of them that yield CONFLICTING answers to the target question:
Case a: b = -13, in which case b IS negative
Case b: b = 0, in which case b is NOT negative
Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT
Statement 2: b³ < 13
There are several values of b that satisfy this condition. Here are two:
Case a: b = -1, in which case b IS negative
Case b: b = 0, in which case b is NOT negative
Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT
Statements 1 and 2 combined
There are still several values of b that satisfy BOTH conditions. Here are two:
Case a: b = -13, in which case b IS negative
Case b: b = 0, in which case b is NOT negative
KEY TAKEAWAY: 0 is a multiple of all integers
Since we cannot answer the target question with certainty, the combined statements are NOT SUFFICIENT
Answer = E
Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
