After reading the question stem of a data sufficiency question, you choose one statement, determine if it’s sufficient to answer the question asked, and then do the same for the other statement. Only after evaluating both statements separately do you consider them together – that is if you haven’t already determined the answer to be (A) or (D).
However, by looking at both statements you can sometimes eliminate a few answer choices right away. These shortcuts occur when the statements contain identical or repeated information, and if you know what to look for, you can save time and increase your chances of selecting the correct answer.
Let’s look at three such examples.
The statements are identical
Look at the following two statements on a hypothetical DS question:
1) 2x + 16y = 30
2) 6x + 8y = 15 + 5x
You may notice some similarities between the two statements. If you simplify the second statement by subtracting 5x from both sides, you get x + 8y = 15, which is the same as the first statement if you multiply both sides by 2. Therefore, the statements are identical.
The only possible answer choices are (D), that each statement alone is sufficient, or (E), that the statements together are not sufficient. The answer cannot be (A) or (B) because the same statement cannot be both sufficient and insufficient, and (C) is also incorrect because combining identical statements together can’t suddenly make them sufficient to answer the question asked.
One statement is derived from the other
Statements do not need to be identical in order to contain overlapping information. In some cases, one statement is an axiom of the other. Here’s an example:
1) q is positive
2) q is a prime number
Note that prime numbers by definition are positive, so statement (2) also informs us that q is positive as well as a prime number. This overlapping information is important because if the fact that q is positive is sufficient to answer the question asked, then the fact that q is a prime number is also sufficient. Therefore, the answer cannot be (A) since (A) states that only statement (1) is sufficient. Also the correct answer cannot be (C). Since one statement can be derived from the other, combining statements does not add new information and cannot determine sufficiency.
One statement repeats information
A third shortcut exists when one statement repeats information from the stem or gives you information you already know. Let’s say a question stem wants you to find the perimeter of a triangular enclosure. The two statements may be something like this:
1) Any two sides of the enclosure is longer than the third side.
2) The longest side is 8 feet longer than the shortest side.
Does the first statement sound familiar? It should because it’s simply repeating the triangle inequality theorem, which states that any two sides of a triangle must add up to a length greater than the third side.
Since statement (1) is an already established fact, it adds nothing to the problem, and therefore has no effect on sufficiency or insufficiency. Therefore you can eliminate any answer choice that involves statement (1), so in this case the answer must be either (B) or (E).