Official Guide Explanation:
Data Sufficiency #98

Background

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Solution and Metadata

Question: 98
Page: 160
Difficulty: 5 (Moderate)
Category 1: Arithmetic > Real Numbers >

Explanation: Statement (1) is insufficient. If yz < 0, then z is less than zero only if y is greater than zero. (The product of two numbers is negative only when exactly one of the numbers is negative.) If xy > 0, y is only greater than zero when x is greater than zero, so z is only less than zero when x is greater than 0. Statement (2) is insufficient, as it contains nothing about z.

Taken together, the statements are sufficient. (1) establishes that if x is greater than 0, z is less than 0. (2) tells you that x is, indeed, greater than zero. Choice (C) is correct.

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