Solving Data Sufficiency through its weakest point

It is a common axiom that the best strategy in any competition is to attack your opponent at his weakest point. If you’ve been studying for the CAT for any length of time, you’ve probably noticed that not all Data Sufficiency statements are created equal. At times the statements are mind-bendingly complex. Other times we can evaluate a statement almost instantaneously, without needing to simplify or calculate.

Anytime you’re confronted with a question that offers one complex statement and one simple statement, you’ll want to attack the question at its weakest point and start with the simpler of the two. Evaluating the easier statement will not only allow you to eliminate some wrong answer choices, but will offer insights into what might be happening in the more complex statement. (And generally speaking, whenever you’re confronted with this dynamic, it is more often than not the case that the complex statement is sufficient on its own.)

Let’s apply this strategic thinking to a complex-looking official problem*:

Data Sufficiency 1

You can see immediately that the first statement is a tough one. So let’s start with statement 2. In natural language, it’s telling us that ‘x’ is less than 5 units away from 0 on the number line. So x could be 4, in which case, the answer to the question “Is x >1?” would be YES. But x could also be 0, in which case the answer to the question would be NO, x is not greater than 1. So statement 2 is not sufficient, and we barely had to think. Now we can know that the answer cannot be that 2 Alone is sufficient and it cannot be Either Alone is sufficient.

Now take a moment and think about this from the perspective of the question writer. It’s obvious that statement 2 is not sufficient. Why bother going to the trouble of producing such a complex statement 1 if this too is not sufficient? This isn’t to say that we know for a fact that statement 1 will be sufficient alone, but I’m certainly suspicious that this will be the case.

When evaluating statement 1, we’ll use some easy numbers. Say x = 100. That will clearly satisfy the statement as (100+1)(|100| – 1) is greater than 0. Because 100 is greater than 1, we have a YES to the question, “Is x >1?” Now the question is: is it possible to pick a number that isn’t greater than one, but that will satisfy our statement?  What if x = 1? Plugging into the statement, we’ll get (1+1)(|1| – 1) or (2)*(0), which is 0. Well, that doesn’t satisfy the statement, so we cannot use x = 1. (Note that we must satisfy the statement before we test the original question!) What if x = -1? Now we’ll have: (-1+1)(|-1| – 1) = 0. Again, we haven’t satisfied the statement. Maybe you’d test ½. Maybe you’d test -3. But you’ll find that no number that is not greater than 1 will satisfy the statement. Therefore x has to be greater than 1, and statement 1 alone is sufficient. The answer is A.

Alternatively, we can think of statement 1 like this: anytime we multiply two expressions together to get a positive number, it must be the case that both expressions are positive or both expressions are negative. In this statement, it’s easy to make (x+1) and (|x| – 1) both positive. Just pick any number greater than 1. However, as mentioned in the previous paragraph, we can immediately see that x=1 will make the second term 0, and x = -1 will make the first term 0. Multiplying 0 by anything will give us 0, so we can rule those options out. Moreover, we can quickly see that any number between -1 and 1 (not inclusive) will make (|x| – 1) negative and make (x+1) positive, so that range won’t work. And any term less than -1 will made (x+1) negative and (|x| – 1) positive, so that range won’t work either. The only values for x that will satisfy the condition must be greater than 1. Therefore the answer to the question is always YES, and statement 1 alone is sufficient to answer the question.

The takeaway: this question became a lot easier once we tested statement 2, saw that it obviously would not work on its own, and became suspicious that the complex-looking statement 1 would be sufficient alone. Once we’ve established this mindset, we can rely on our conventional strategies of picking numbers or using number properties to prove our intuition. Anytime the CAT does you the favor of giving you a simple-looking statement, take advantage of that favor and adjust your strategic thinking accordingly.