Brian Galvin is the Director of Academic Programs at Veritas Prep, where he has taught GMAT classes since 2003. The co-author of Veritas Prep’s lesson books and co-host of its Veritas Prep on Demand video lesson series, Galvin holds degrees in both business and education from the University of Michigan, and spends his free time dreaming up Data Sufficiency problems while swimming, biking, and running along the coast in Santa Monica, California.
Here are his three tips for conquering the GMAT’s Data Sufficiency portion:
- Beware of typical assumptions. When you can solve an inequality to break down to 0 < x < 2, that doesn’t mean that x = 1. You can’t assume that x is an integer, but it’s typical for your mind to do so while you’re thinking quickly. Similarly, if x-squared = 16, x could be 4…but it could be -4. Make sure that you don’t commit “easy” errors like this, particularly as questions get more complex and you set your brain to autopilot for the simpler calculations and conclusions. Write down a checklist – we suggest “Negative, Noninteger, Zero” to ensure that you check for the potential for some of the not-as-obvious numbers that your mind tends to overlook.
- Manipulate algebra…even in the question stem.. Data Sufficiency questions are asking you whether you can prove a conclusion, and to prove a conclusion you need to leverage facts. For example, if a question asks “Is x > y?”, and statement 1 allows you to solve for x (x = 7, say) and statement 2 allows you to solve for a range of y (y < 4), then you can leverage those facts (7 is greater than any possible value of y, so x is definitely greater than y) to answer the question.
But here’s where it can get sneaky – the GMAT often supplies you with facts that are more like riddles. They’re not as direct as they could be. So a question might ask: Is x > y?. And statement 1 might say: (x^2 – y^2) > 0, and statement 2 might say that x and y are both positive integers. You now need to manipulate that algebra – on the surface you still may not have enough information. But if you factor out statement 1 to find that: (x + y)(x – y ) > 0, now you’re getting somewhere. That mean that those two parentheticals are either both positive or they’re both negative. And since statement 2 tells you that x + y must be positive, then that means that x – y must also be positive. And if x – y > 0, then, doing the algebra, you’ll find that x > y.
The lesson? In order to leverage statements and questions, you have to take the inconvenient statements you’re given and manipulate the algebra to make them more useful. And if a question stem asks something like “Is x/y = z?”, you may want to ask that same question but without the division (is x = yz?), having performed the algebra on the question stem, too.
- Pay attention to details, and question everything. Suppose you’re asked the same question in two different ways, and statement 1 in both cases is the same:
x represents the number of children on a field trip. What is the value of x?
x represents the number of kilograms of grain in a bag. What is the value of x?
statement 1: 3 < x < 5
For the first question, the statement is sufficient. You can’t have 3.5 or 4.9 children, so the answer must be 4. But for the second question, it’s not sufficient. You CAN have 3.5 kilograms of grain…but you could just as easily have 4. The difference between these statements is subtle, mainly because students are so often in a hurry to “do math” that they ignore what they consider to be the backstory. As a general rule, if a question or statement contains an entire sentence, relationship, or clause that you don’t use, you’re probably missing something. Data Sufficiency questions are less than half “math questions” – they’re mostly logic puzzles. So don’t let yourself get in the habit of just-doing-math – make sure you’re also reading critically for logic.
–Alanna Stage, @AlannaTweets