Practice GMAT Data Sufficiency Question
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Is 13N a positive number?
 21N is a negative number
 N^{2} < 1
Correct Answer: A
 Simplify the question:
Since multiplying a number by 13 does not change its sign, the question can be simplified to: "is N a positive number?" 
Evaluate Statement (1) alone.
 Write out algebraically:
21N = negative
21N = positive {divided by 1}
N = positive  Since N is a positive number, 13N will always be a positive number.
 Statement (1) alone is SUFFICIENT.
 Write out algebraically:

Evaluate Statement (2) alone.
 Any time you are dealing with a number raised to an even exponent, you must remember that the even exponent hides the sign of the base (e.g., x^{2} = 16; x = 4 AND 4).
 Solve the inequality:
N^{2} < 1
1 < N < 1 {take the square root, remembering that there is a positive and negative root}  Since N can be both positive (e.g., .5) or negative (e.g., .5), Statement (2) is not sufficient.
 Statement (2) alone is NOT SUFFICIENT.
 Since Statement (1) alone is SUFFICIENT but Statement (2) alone is NOT SUFFICIENT, answer A is correct.
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