GMAT Data Sufficiency: Follow Your Process - Part 1
Data Sufficiency problems are fantastic because you can avoid doing lots of math calculations! Data Sufficiency problems are really annoying because theyre so tricky; its very easy to fall into a trap. :(
So how do we take advantage of the benefits of DS without falling prey to the test-writers tricks?
Try this GMATPrep problem from the free problem sets and then well talk!
*If both x and y are nonzero numbers, what is the value of [pmath]y/x[/pmath]?(1) x = 6
(2) [pmath]y^2=x^2[/pmath]
(Wondering where the answer choices are? I havent put them here. If you haven't done enough DS to know what the answer choices are already, then dont start with this article. Start here.)
Got your answer? Okay, lets get started.
Glance at the problem. Its DS! Draw a T on your scrap paper and jot down the relevant info as you read. Also note that the information looks pretty straightforward in both the question stem and the statementsso work methodically. Dont get sloppy and make a careless mistake.
Now what? You could go straight to the statements and start working, but reflect a moment.
What would be sufficient to answer that question?
If they tell me the values for x and y individually, then I can answer the question. Or if they tell me the value of the quantity or combination [pmath]y/x[/pmath], then I can also answer the question.
Okay, lets do this.
That first statement tells me x but nothing about y, either alone or in relation to x. Thats not sufficient. What about the second one?
Careful! Trap number 1 is lurking. Forget what you saw in statement (1). It doesnt exist.
Next, can you manipulate that equation? Lets see.
[pmath]y^2/x^2=1[/pmath]
Does that mean that just plain[pmath]y/x[/pmath]is also 1? Maybe. But not necessarily! Heres trap number 2.
Either one of the base variables could be negative. For instance, x could be 3 and y could be -3. If so, then the answer would be 1, not +1. This one is really tempting, but its not enough on its own. This statement is not sufficient.
What happens when we put them together?
Finally, this one must be sufficient, right?
Nope! This is trap #3. Its true that x = 6, but y could still be +6 or 6, so the answer is either 1 or 1. Thats not sufficient; for a value question, there has to be just one answer.
The correct answer is (E).
The math on this problem is not super-advanced. The question is definitely a beautiful question thoughat least, from the point of view of the test writer. :) They set two traps to steer you towards choice (B) and a third trap to steer you towards choice (C), all in a problem that looked pretty straightforward at first glance. Now imagine how mentally fatigued youre going to get as the test progresses and you can see how easy it might be to fall into one of these traps without even realizing it.
Check back next time when we'lladd a few more process layers that are specific to DS.
Key Takeaways for Data Sufficiency:
(1) Have a consistent process and stick to it. The process I describe in this article is the one I use, but you dont have to use mine. Whatever you do use, just make sure that it clearly delineates the various steps you need to take and that you always follow the same steps, every time.
(2) Dont rush when you think something is easy! Speed causes careless mistakes. When you know how to do something, make sure you get that point. Save rushing for the ones that are really hard! Chances are good that well get those wrong anyway; we might as well get them wrong faster.
* GMATPrep questions courtesy of the Graduate Management Admissions Council. Usage of this question does not imply endorsement by GMAC.