mirhaque wrote:

Is this a GMAT-ly correct question? What's the answer halle?

That is an implication, not a truth, about GMAT questions. My friend took GMAT the other day; he got a question with a contradiction.

Lets deal with a contradiction a bit!

1. x is an integer, what is the value of x?

A. (x-5)(x-4) = 0

B. (x-4)(x+6) = 0

2. x is an integer, what is the value of x?

A. (x-5)(x-4) = 0

B. (x+5)(x+4) = 0

If we were to go by the slogans of this board, the second question would not be GMAT-ly correct question!

1a. x belongs to A = {4, 5}

1b. x belongs to B = {4, 6}

AB = {4}, a set with only one member, hence sufficient.

2a. x belongs to C = {4, 5}

2b. x belongs to D = {-4, -5}

CD = { } = null set, hence insufficient.

In other words, the question is asking whether a solution set with *only one* member exists. A null set--a consequence of contradiction--has no members!