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Statement 1: x + a > x - a This statement doesn't include any information about y, so there's no way to answer the target question. Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT
Statement 2: ax > ay Some students will divide both sides by a and incorrectly conclude that x > y. However, before we divide by a variable, we must ensure that the variable is EITHER positive OR negative, because if we divide by a negative value, we must reverse the direction of the inequality, and if we divide by a positive value, the direction of the inequality stays the same. As it stands, we don't know whether a is positive or negative.
To see what I mean, consider these values of a, x and y that satisfy the given condition: Case a: a = 1, x = 3 and y = 2, in which case x > y Case b: a = -1, x = 2 and y = 3, in which case x < y Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT
Statements 1 and 2 combined Statement 1 tells us that x + a > x - a Add a to both sides to get: x + 2a > x Subtract x from both sides to get: 2a > 0 Divide both sides by 2 to get: a > 0. In other words, a is POSITIVE
Statement 2 tells us that ax > ay Now that we know that a is POSITIVE, we can take ax > ay and safely divide both sides by a to get: x > y PERFECT! Since we can answer the target question with certainty, the combined statements are SUFFICIENT
==> In the original condition, there are 2 variables (x, y) and in order to match the number of variables to the number of equations, there must be 2 equations. Since there is 1 for con 1) and 1 for con 2), C is most likely to be the answer. By solving con 1) and con 2), from con 1), you get a>-a, 2a>0, or a>0, and from con 2), you get ax>ay, and the inequality sign doesn’t change even if you divide both sides by a because since a>9, you get x>y, hence yes, it is always sufficient.
Therefore, the answer is C. Answer: C
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Statement 1: x + a > x - a a + a > x - x 2a > 0 a > 0 a is POSITIVE That's all this statement tells us. Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT.
Statement 2: ax > ay ax - ay > 0 a(x - y) > 0 Case a: a > 0 and (x - y) > 0, in which case x > y Case b: a < 0 and (x - y) < 0, in which case x < y Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT.
Statements 1 and 2 combined Statement 1 tells us that a is POSITIVE
Statement 2 tells us that, if a > 0, x > y
Since we can answer the target question with certainty, the combined statements are SUFFICIENT
Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).
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