Practice GMAT Data Sufficiency Question

Is 13N a positive number?

A)	Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
B)	Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
C)	BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
D)	EACH statement ALONE is sufficient.
E)	Statement (1) and (2) TOGETHER are NOT sufficient to answer the question asked, and additional data are needed.

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.
1. Write out algebraically:
  -21N = negative
  21N = positive {divided by -1}
  N = positive
2. Since N is a positive number, 13N will always be a positive number.
3. Statement (1) alone is SUFFICIENT.
Evaluate Statement (2) alone.
1. 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., x² = 16; x = 4 AND -4).
2. Solve the inequality:
  N² < 1
  -1 < N < 1 {take the square root, remembering that there is a positive and negative root}
3. Since N can be both positive (e.g., .5) or negative (e.g., -.5), Statement (2) is not sufficient.
4. Statement (2) alone is NOT SUFFICIENT.
Since Statement (1) alone is SUFFICIENT but Statement (2) alone is NOT SUFFICIENT, answer A is correct.