prab wrote:
Is a > b ?
(1) ca > cb
(2) 3a > 3b
Target question: Is a > b ? Statement 1: ca > cb The temptation here is to divide both sides by c to get a > b, but this approach has a major flaw, since we don't know whether c is POSITIVE or NEGATIVE
If c is NEGATIVE, then we must REVERSE the direction of the inequality after dividing by c (see the video below for more on this)
Alternatively, we can demonstrate this by examining some values of a, b and c that satisfy statement 1:
Case a: a = 1, b = 0 and c = 1. In this case, ca > cb becomes (1)(1) > (1)(0), which is true. Here, the answer to the target question is
YES, it is the case that a > bCase b: a = 0, b = 1 and c = -1. In this case, ca > cb becomes (-1)(0) > (-1)(1), which is true. Here, the answer to the target question is
NO, it is NOT the case that a > bSince we cannot answer the
target question with certainty, statement 1 is NOT SUFFICIENT
Statement 2: 3a > 3bSince 3 is POSITIVE, we can safely divide both sides of the inequality by 3 to get: a > b
So, the answer to the target question is
YES, it is the case that a > bSince we can answer the
target question with certainty, statement 2 is SUFFICIENT
Answer: B
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