Using GRE Probability Questions to Catch Up

Do you struggle with the GRE’s time constraints? Do you have difficulties solving probability questions?

Yes? Fantastic! In this article, we’ll examine how probability questions can be a great opportunity to make up lost time.

Consider the following scenario:

It’s test day, and halfway through one of the quantitative reasoning sections, you find that you’re minutes behind AND a probability question has just appeared on the screen.

At this point, your options are:

1)    Work faster on the remaining questions (and risk making careless mistakes)

2)    Skip the question, and return to it later

3)    Guess and immediately make up the lost time  

Of course, all three options are less than perfect, but I’ll argue that option #3 (guessing) is the best approach, especially since we’re dealing with a probability question.

To illustrate this, try to answer the following question in 20 seconds or less:

From a group of 5 employees (Ann, Bea, Cam, Don and Ed), 2 people are randomly selected to attend a conference in Hawaii.  What is the probability that Ann and Bea are both selected?

(A) 0.1

(B) 0.2

(C) 0.25

(D) 0.35

(E) 0.65

If you’ve already identified probability as one of your weaknesses, and if you typically fall behind time-wise, this question is a gift because, within a matter of seconds, you should be able to eliminate 2 or 3 answer choices, make an educated guess and move on to a question that you have a better chance of solving.

The strategy relies on the fact that most people have an innate ability to judge the relative likelihood of an event. For example, even though we may not be able to calculate the exact probabilities, we know that rolling a die four times and getting a 6 each time is less likely than rolling 2 dice and getting a sum of 7.

So, for the question about the Hawaii conference, you can quickly use some intuition to eliminate answer choices that don’t feel right.

To begin, you might ask, “Is the probability of selecting Ann and Bea greater than 0.5 or less than 0.5?” If it feels less than 0.5 (which it is), eliminate E.

Of course, eliminating one measly answer choice doesn’t help a whole lot, so you’ll need to be much more aggressive. So, you might ask, “Does it seem very unlikely that Ann and Bea are both selected, or does it seem somewhat unlikely? Your answer will allow you to eliminate additional answer choices.

If it seems very unlikely that Ann and Bea are both selected, you might eliminate C, D and E, leaving yourself with a 50-50 chance of guessing the correct answer (all within seconds of reading the question!). If you’re less aggressive, you might eliminate just D and E. That’s still fine. Just remember that the goal is not to ensure that you correctly answer the question; the goal is to make up some much-needed time AND maximize your chances of guessing the correct answer.  

Finally, I’d like to remind you that, if you do somehow manage to make up all of your lost time, you can always return to the question later and try it again.

By the way, the correct answer to the question is A (0.1).