BrentGMATPrepNow
What is the value of x² + 4x - 5?
(1) |x - 2| = 1
(2) |x + 2| = 3
I call this question a fool me once question (FMO). The mistake many students will make is to conclude that, in order to answer the target question, we need only determine the value of x. So, when those students see that each statement yields two different possible value of x, they (incorrectly) conclude that each statement alone is insufficient. Remember that the target question is not asking us to find the value of x; it's asking us to find the value of x² + 4x - 5, and we can't answer that question without first plugging values of x into the expression.
Once you make this mistake, you're less likely to make it again (thus the FMO designation)Target question: What is the value of x² + 4x - 5? Statement 1: |x - 2| = 1 This means EITHER
x - 2 = 1 OR
x - 2 = -1Let's examine each possible case...
Case a: If x - 2 = 1, then x = 3. In this case, the answer to the target question is
x² + 4x - 5 = 3² + 4(3) - 5 = 16Case b: If x - 2 = -1, then x = 1. In this case, the answer to the target question is
x² + 4x - 5 = 1² + 4(1) - 5 = 0Since we can’t answer the
target question with certainty, statement 1 is NOT SUFFICIENT
Statement 2: |x + 2| = 3This means EITHER
x + 2 = 3 OR
x + 2 = -3Let's examine each possible case...
Case a: If x + 2 = 3, then x = 1. In this case, the answer to the target question is
x² + 4x - 5 = 1² + 4(1) - 5 = 0Case b: If x + 2 = -3, then x = -5. In this case, the answer to the target question is
x² + 4x - 5 = (-5)² + 4(-5) - 5 = 0So, it MUST be the case that
x² + 4x - 5 = 0Since we can answer the
target question with certainty, statement 2 is SUFFICIENT
Answer: B
Cheers,
Brent