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Top 10 Tips for the GMAT Data Sufficiency Section

Posted in Test Prep on September 22, 2010 by

If you’re just starting your GMAT prep (or even if you’ve been at it for a little while), the Data Sufficiency section can be tricky. After all, who ever heard of a math problem that you don’t actually have to solve?

The good news is that with some strategic practice, you too can train yourself to think like a DS whiz. To master this section, start by becoming familiar with the structure of DS questions and the concepts they most commonly test.

Here are some concrete tips to get you on track:

1. Be very familiar with the answer choices.

No excuses: On Data Sufficiency, they’re always the same! Know in the blink of an eye what choice C is. On test day, if you find that Statement 1 is insufficient, be able to cross out choices A and D without hesitation.

2. Write down what you absolutely need in order to find certain quantities.

Each statement alone will be sufficient if both of the statements on their own contain all the information necessary to answer the question. The statements will be sufficient together if they contain every piece of necessary information between them. Take the area of a parallelogram: Do you need to know every side length to determine the area? If you have every side length, can you find the area?

3. Don’t look at the statements together.

Statement 2 may tell you that x is negative, but that fact has no bearing on Statement 1 when viewed by itself. Explore all the possibilities offered by each statement individually. If you’ve scrutinized Statement 1 and found it sufficient, be equally merciless when it comes to Statement 2.

4. Important information is often buried in the prompt.

Don’t pay so much attention to the statements that you forget the rest of the question. Often, half the information that you need is in the set-up.

5. Know when it’s actually necessary to solve single-variable equations.

If the question asks for the value of x and you whittle the problem down to an equation like 305x = 2(500) – 10205, don’t waste your time solving for x! It’s only important to know that you COULD solve if you wanted to. Remember, all linear one-variable equations have a unique solution, but quadratic equations—equations with an x^2 term—can have zero, one, or two solutions.

6. Know when it’s necessary to solve a system of equations.

Again, you never need to solve a DS problem—you only need to know that you could. A system of n independent linear equations with n variables can be solved for ALL of the n variables. The key word here is “independent”: Equations are independent if they’re not multiples of one another. For example, y = 2x and 3y = 6x are NOT independent equations because the second equation is just three times the first. If on test day you don’t feel comfortable declaring that a system of equations is solvable, get the system down to one single-variable equation and then reassess.

7. Study prime factorizations and divisibility.

Although any GMAT math concept is fair game on the DS section, prime factorization shows up frequently and reliably. If x is divisible by 15, will x^2 be divisible by 27? What about x^3?

8. Study overlapping sets.

Be comfortable representing these overlapping sets with Venn diagrams. This topic is a DS favorite. A statement like, “The number of widgets that were not made in Factory A or Factory B is three times greater than the number of widgets that were made in Factory B” can be difficult to unpack in the heat of the moment. Train yourself to answer questions about sets methodically and quickly.

9. Remember that only 2 out of the 5 answer choices involve looking at both statements TOGETHER.

This means that there’s a 60% chance that the correct answer will treat the statements on an individual footing. It can be tempting to use all the information the problem provides, but keep these odds in mind. Choices C and E, as a group, are 20% less likely to be correct than choices A, B, and D, as a group.

10. Be on the lookout for statements that give no new information.

The area of a square, for instance, contains just as much information as the side length of the square. If you know the area, you can find the side length; conversely, if you know the side length, you can find the area. Often on the DS section, Statement 2 will just be a repackaging of the same information provided by Statement 1.