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GRE Probability Questions

If you’re like most students, you probably struggle with the GRE’s time constraints, and you probably have difficulties with probability questions.

Great! In this article, we’ll examine how probability questions can provide you with a convenient opportunity to make up lost time.

To set this up, please consider the following scenario:

It’s test day, and halfway through one of the math sections, you find that you’re 2 minutes behind.

At this point, you have two options:

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

2)   Guess on one of the questions and immediately make up the lost time (but risk guessing the wrong answer)

Both options are less than ideal, but I’ll argue that option #2 is better than option #1, especially if you encounter a probability question.

To illustrate this, answer the following question within 20 seconds:

From a group of 5 managers (Joon, Kendra, Lee, Marnie and Noomi), 2 people are randomly selected to attend a conference in Las Vegas.  What is the probability that Marnie and Noomi are both selected?

(A) 0.1

(B) 0.2

(C) 0.25

(D) 0.4

(E) 0.6

If you’ve already identified probability as one of your weaknesses, and if you typically fall behind time-wise, this question is a gift. You should be able to eliminate 2 or 3 answer choices and make an educated guess within seconds of reading the question.

The elimination strategy relies on the fact that most people have an innate ability to judge the relative likelihood of an event. So, for the Las Vegas question above, you can use your intuition to eliminate answer choices that just don’t feel right.

 

To begin, you might ask, “Is the probability of selecting Marnie and Noomi greater than 0.5 or less than 0.5?” If it feels less than 0.5 (which it is), you can eliminate E.  Of course, you’ll want to eliminate more than 1 answer choice, so you’ll need to be more aggressive. You might ask, “Does the event seem very unlikely or a little unlikely? Your answer will allow you to eliminate additional answer choices.

If you feel that the probability seems very unlikely, you might eliminate C, D and E, leaving yourself with a good 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. Remember that the goal here is not to ensure that you correctly answer the question; the goal is to make up your 2 minutes and maximize your chances of guessing the correct answer.

Please note that this guessing strategy can also be used if you typically run out of time on the math sections, and you need a way to give yourself a buffer. Just remember that probability questions are the best for this (counting questions are pretty good, too).

To illustrate how GRE probability questions are better than other question types for guessing, try answering this next question within 20 seconds:

Gita is 12 years older than Harvey. In 4 years, Gita will be twice as old as Harvey. How old will Harvey be in 3 years?  

(A) 5

(B) 11

(C) 15

(D) 19

(E) 23

Although this question a little easier than the Las Vegas question, it’s a bad candidate for guessing (and recapturing lost time), since we really can’t use our intuition to legitimately eliminate answer choices.

In my next article, I’ll talk more about how we can further eliminate answer choices in probability questions before we guess. In that article, I plan to continue discussing the Las Vegas question, so I won’t tell you the answer just yet. I will, however, tell you that the answer to the Gita/Harvey question is B.

 

By the way, students who use Magoosh GRE improve their scores by an average of 8 points on the new scale (150 points on the old scale.) Click here to learn more.

5 Responses to GRE Probability Questions

  1. Ameer Hussein March 1, 2013 at 2:54 pm #

    Hello sir
    For the question

    Gita is 12 years older than Harvey. In 4 years, Gita will be twice as old as Harvey. How old will Harvey be in 3 years?
    I answer it as bellow /

    G/ for old of Gita , H/ for old of Harvey!

    G=H+12
    G+4=2H

    Sub 2 in 1
    We get G= 28

    H= 16

    H after 3 years will be 16+ 3 = 19

    Answe is C ,

    I hope you can explain to me

    Best

    • Chris Lele
      Chris Lele March 5, 2013 at 1:40 pm #

      Hi Ameer,

      The step you missed was in translating the “In 4 years, Gita will be twice as old as Harvey”.

      You added 4 to Gita but not to Harvey. Therefore, the equation should read

      G+4 = 2(H+4),

      G+4 = 2H + 8…by using substitution, you should get to answer (B).

      Hope that helps!

    • Nikita Saharia August 25, 2015 at 12:04 pm #

      Let Harvey’s present age be x
      Then, Gita’s present age = x + 12

      After 4 years Gita’s and Harvey’s age will be x+12+4 and x+4 respectively.

      Thus, the equation is as follows:

      x + 12 + 4 = 2( x + 4)

      x + 16 = 2x + 8

      sub -x from both sides,

      16 = x + 8

      sub -8 from both sides,

      x = 8

      Hence the present age of Harvey is 8 yrs, and three years from now he will be 11 yrs.

      Thus choice B is the correct ans.

  2. shara September 7, 2011 at 9:09 am #

    Could you please explain the correct strategy used to find the correct answer to the Las Vegas question? I still cannot figure it out and my math skills are not the best.


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