GMAT Probability In Perspective

March 8, 2010

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If there's one thing that worries GMAT test-takers more than anything else, it's probability. It's as if there's a big warning sign on the inside cover of every GMAT prep book ever printed telling students that they won't get the score they want until they spent 50 hours studying probability.

Let me tell you: it isn't true.

Probability, as a field of study, can be very hard: you can take entire graduate-level mathematics courses on nothing but probability. Surf and you'll find plenty of probability textbooks covering topics that will make your head spin. But, of course, all of that stuff isn't on the GMAT. As usual, what's on the test are a few basic subjects.

Best of all, it's not hard to figure out what those topics are. The questions in The Official Guide to GMAT Review are the best indicator of what you'll see on the real thing, and there are only a handful of probability questions in there. (No, the GMAT doesn't "save" the really hard stuff for the test itself.)

If you can do the probability questions in the Official Guide, you can handle all the probability you need on the GMAT itself. It's that simple. Don't let anyone tell you otherwise.

In fact, there are exactly four fundamental concepts you need to nail all but the very hardest of GMAT probability questions:

  • Probability equals the number of desired outcomes divded by the number of possible outcomes.
  • The probability of two discrete events occuring (say, flipping a coin twice and getting two heads) is the product of the two individual probabilities.
  • The probability of getting one or another result (say, removing a jellybean from a bag and getting a red jellybean or a blue jellybean) is the sum of the two individual probabilities.
  • The probability of something not happening is one minus the probability that it does happen. (And vice versa.)

That's it.

Of course, the GMAT has plenty of twists they can put on those basic concepts. But in that regard, probability is no different from, say, inequalities. In fact, I'm convinced that several topics (including inequalities) are harder to grasp at the highest levels than probability.

So, master a few basic rules, do a few practice problems, and then turn your attention to stuff that really does matter. Just because test-prep companies feed you dozens of incredibly difficult probability questions doesn't mean that's what you'll see on the test. It only means that they know what you'll pay for.



About the author: Jeff Sackmann has written many GMAT preparation books, including the popular Total GMAT Math, Total GMAT Verbal, and GMAT 111. He has also created explanations for problems in The Official Guide, as well as 1,800 practice GMAT math questions.

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