Pick numbers carefully and strategically on GMAT Data Sufficiency Yes/No questions.
One of the hallmark points of confusion on GMAT Data Sufficiency is the dreaded Yes/No question.
In a Value question, such as “What is the value of x?” the question of sufficiency is a familiar one: if you can solve for x, you have sufficiency. But in a Yes/No question, especially when variables are involved, finding a solid answer can be a much cloudier process.
Sample GMAT Data Sufficiency Yes/No Question
The best way to clear this fog is with a concrete example. Let’s look at this Data Sufficiency question, along with its first statement:
Is x positive?
(1) x^2 > 1
Is Statement (1) sufficient to answer the question? Unless you have a comprehensive understanding of the underlying Number Properties at work here, your first reaction to this statement is likely to try out different numerical values for x, because working with real numbers instead of variables will be a much more comfortable place for most of us. You are free to try out any value for x, but your first consideration in checking this statement should be that the number you pick is permissible, according to the statement. If it is not, don’t consider the number as an example.
Picking a Permissible Number
Is zero a permissible number to use here? Well, if x = 0, then x^2 is also 0, and this statement tells you that x^2 has to be greater than 1. You must take the statements as true, so zero is NOT a number you can use here (not permissible).
How about x = 2? That is in permissible territory, because 2^2 = 4, and 4 > 1. But even that is only half the battle. Now that you know x = 2 is a permissible example, you have to see what answer it yields to the original question, “Is x positive?” Since 2 is a positive number, the answer here is “Yes.”
Sufficiency on Yes/No Data Sufficiency Questions
Now there’s one example in the bank, and you know that, given the information in this statement, the answer to the question can be “Yes.” But is that enough to declare sufficiency? Unfortunately, it is not. If this statement is sufficient to answer the question, it will give you a consistent Yes or No answer. You know now that the answer could be Yes, but could it also be No?
Well, if the answer could be No, then that would mean x could be negative or zero. You’ve already seen that x can’t be zero (because it’s not permissible, remember?), so what about x being negative? Let’s take the flip side of the other example and try x = -2. It would certainly answer the question stem with a No, but is it permissible?
Remember, the statement mandates that x > 1. Working a little calculation, (-2)^2 equals (-2)(-2), and since the product of two negative numbers is a positive number, x^2 = 4 when x = -2. So this second example is permissible after all, and it answers the question “Is x positive?” with a resounding “No.” Since you have answered the question with a potential “Yes” (when x = 2) and a potential “No” (when x = -2), this statement is actually insufficient in the end; further information is required to determine whether or not x is positive.
The Takeaway
As you see, it is absolutely necessary to remember what must be assumed as true (the statements) and what may or may not be true (the question stem) when Picking Numbers in these types of problems.
While this specific example is not as challenging as some, and you may have logically thought through it with number properties rules from the outset, this thought process is vital to learn for these types of questions, and will be most helpful with the most challenging questions, where you cannot gather the potential scenarios quickly at a glance without doing some scratch work. When the Yes/No monster rears its ugly, convoluted head, never forget when picking numbers: First permissible, then Sufficient!
Test your GMAT skills. Sign up for a free GMAT practice test.
The post Picking Numbers on GMAT Data Sufficiency: Permissible vs. Sufficient appeared first on Kaplan GMAT Blog.
