10 Very Specific Ways to Study for the Data Sufficiency Section

by on June 10th, 2010

numbers1. Be very familiar with the answer choices.

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. Get in the habit of writing down what you absolutely need in order to find certain quantities.

Each statement will be sufficient if it contains at least every piece of necessary information. The statements will be sufficient together if they contain every piece of necessary information between them. Take the area of a parallelogram, for example. Do you need to know every side length? If you have every side length, can you find the area?

3. Train yourself not to 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. Don’t let it off the hook just because it doesn’t contradict Statement 1.

4. Remember that 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, and be comfortable representing these overlapping sets with Venn diagrams.

This topic is another 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. If you train yourself, you can learn how to answer questions about sets methodically and quickly.

9. Only 2 out of the 5 answers choices involve looking at both statements TOGETHER.

So there’s a 60% chance that the correct answer will treat the statements on an individual footing. Though it’s tempting to use all the information the problem provides, you should 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.

6 comments

  • Amazing Article.
    Thanks a lot

  • Great article, however I have to disagree with #9 implying that more often than not the answer is ABD rather than CE. Often times either A or B as a standalone statement has absolutely zero chance of answering the question. In those cases, both that particular choice and D can be eliminated immediately.

    Anecdotally, I actually think the majority of answers belong to group CE. But if the question is super difficult it tends to be ABE.

    If I had to guess the overall distribution of answers (someone like Patrick of GMATFix with a huge database could probably provide real evidence) from most frequent to least frequent, it would be:

    C
    E
    D
    A/B

  • In addition to tip no.6, although you can always solve each of the n variables when you know n independent equations, don't quickly justify that a statement is insufficient just because you have fewer independent equations than the number of variables involved.

    Very often you find a statement sufficient to give you a definite answer on a DS question prompt even when you can't solve each variable given in the equations.

  • Excellent post. It is interesting that I was messing up my DS practice with all the above errors whence I came across this article

  • Great article...

  • I'd just like to let u know how much I learnt from your blog Dugg you.Hope 2 be back fast for some more good stuff

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