GMAT Data Sufficiency Tips

When students consider what aspects of the GMAT stand out more than those of any other standardized test, the question adaptive nature of the test is most likely at the top of the list. Unlike any other test, the GMAT measures your performance on the test, question by question, a feature which necessitates that you answer the previous question before moving on to the next one. But coming in a close second in terms of GMAT uniqueness has the be the Data Sufficiency questions that comprise a little less than half of the Quantitative section of the GMAT.

GMAT Data Sufficiency: What is it?

The GMAT Quantitative Section consists of 31 questions, and you’ll have 62 minutes to complete it in standard time—that’s 2 minutes per question. Though the number of Data Sufficiency questions may vary, they usually account for a little less than half (about 13-15) of the total Quant questions. The remaining Quantitative questions are known as Problem Solving, and these are more like the conventional math questions which require you to perform calculations and come up with a specific solution.

But your online GMAT prep will most likely begin with embracing the Data Sufficiency question, which is unlike anything you’ve ever seen before. Firstly, you don’t have to come up with an answer! You just have to determine if you have enough information to come up with an answer. Secondly, the answer choices are exactly the same for every Data Sufficiency question. In short, the DS question goes beyond simply testing your direct math skills; it assesses your ability to analyze a mathematical problem and decide what information is sufficient to its successful solution. Here are some general tips that will start you on your way.

GMAT Data Sufficiency Tip: Know the Answer Choices by Heart!

Here they are, without variation:

Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient
Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient
Both statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient
EACH statement ALONE is sufficient
Statements (1) and (2) TOGETHER are NOT sufficient

Many folks use the “12TEN” mnemonic to help remember these choices:

1 – Alone, 2 – Alone, Together 1 & 2, Either Alone, or Neither Alone

GMAT Data Sufficiency Tip: Simplify the Stem

The Data Sufficiency question has two parts: the stem and the statements.

The stem is the initial presentation of the information that may contain a fact and is followed by a question. The question itself is of two types:

A value question
- What is the value of x?
- What was the total of Phillipa’s salary and commission last month?
A Yes/No question
- Is x > 70?
- Does Ezra weigh more than Clay?

The statements are just that—facts, taken as truth, and you have to determine if they are sufficient enough (in the combinations presented above) to answer the question part of the stem.

GMAT Data Sufficiency Tip: Resist the Temptation to Do the Full Calculation!

This is the beauty of these questions, but also one of the hardest parts for students to master: only go so far as you have to in order to determine the information is enough (thus, the sufficiency part of the name).

GMAT Data Sufficiency Tip: Remind Yourself to First Consider Each Statement Alone

When you’re considering each statement, remind yourself that the other statement does not exist.

Data Sufficiency Sample Question 1:

What is the average (arithmetic mean) of a and b?

The average (arithmetic mean) of a + 2 and b + 4 is 11.
The average (arithmetic mean) of a, b, and 14 is 10.

Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient
Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient
Both statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient
EACH statement ALONE is sufficient
Statements (1) and (2) TOGETHER are NOT sufficient

Data Sufficiency Sample Question 1 (Explanation)

First, let’s simplify the stem. Getting the average of 2 numbers involves just knowing their sum. To get the average, I don’t need to know what a and b are individually; I just need to know that (a + b)/2 = the average. So, my stem simplifies to what is a + b?
Second, I’m ready to tackle the statements.
- Statement 1 tells me that: (a + 2) + (b + 4) = 2 x 11
  - Do I have to calculate this to determine what a + b is? NO! I simply know that I have enough information to do so. Therefore, Statement 1 Alone => Sufficient!
- Statement 2 tells me that: a + b + 14 = 3 x 10
  - Same as above, do I have to calculate a + b? NO! Statement 2 Alone =>Sufficient!
- Answer D
Granted, this is a more basic example, but you will be thankful for this approach especially on more complex examples.

GMAT Data Sufficiency Tip: Plug In Numbers (If you can’t figure out the algebra)

Quick, think of a number less than 10. Most people respond to this with an all-American positive integer, like 2 or 3 or 9. Most people would not immediately respond with -47 or ½ or 9.873. Fact is, while they are not natural numbers, they are ALL numbers of some kind. Many Data Sufficiency questions test your ability to consider these numbers.

Data sufficiency questions can also be tackled by plugging in numbers that are dictated by the terms of the question stem. For instance, if the stem says, Is |x – y| > |x| – |y|?, you should be aware that absolute value measures a number’s distance from 0. Consequently, you should choose different combinations of number values: 1) when x and y are both positive, 2) both negative, 3) x positive, y negative, and 4) x negative, y positive. This is often the best way to determine sufficiency.

Data Sufficiency: Sample Question 2

Is a³> a²?

a > 0.
a < 1.

Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient
Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient
Both statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient
EACH statement ALONE is sufficient
Statements (1) and (2) TOGETHER are NOT sufficient

Data Sufficiency Sample Question 2 (Explanation)

This particular stem cannot really be simplified and stands as it is as a Yes/No question. What it forces us to consider, however, is can a number raised to the 3^rd power ever be LESS than a number that is raised to the 2^nd power? As discussed above, if we think only in terms of positive integers (excluding 1 for the moment), then the above expression is always true. When we consider those less conventional numbers, though, the expression may not be true.
Statement 1: Let’s plug in some numbers – a has to be positive
- If a = 2, then the answer to the question stem is yes
- If a = ½, then the answer to the question stem is no
- Also, you can consider that if a = 1, then answer is also no
- After plugging in different positive numbers and getting contradictory answers to question stem, I can conclude that this statement is =>
- Eliminate Choices A & D; consider Choices B, C, E
Statement 2: I notice that this statement eliminates all positive integers, including 1. It leaves me with positive fractions and all negative numbers, including negative fractions. Let’s do some plugging in!
- If a = ½, then answer to question stem is no
- If a = – ½, then answer to question stem is no
- If a = -2, then answer to question stem is no
- If the answer is “no” in all categories, then we can assume that Statement 2 is sufficient to answer the question in the stem. (A “no” answer, by the way, to a yes/no question is definitely sufficient.)
- Therefore, Statement 2 ALONE => Sufficient
- Answer B