GRE Math : GRE Quantitative Reasoning

Study concepts, example questions & explanations for GRE Math

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Example Questions

Example Question #1 : How To Find The Union Of A Venn Diagram

In a class of 100 students, 43 play basketball and 37 play baseball. 9 students play both. How many students do not play either sport?

Possible Answers:

29

38

not enough information to answer the question

20

71

Correct answer:

29

Explanation:

In order to determine how many students are not enrolled in a sport, we must first determine how many students are. The simplest way to do this is to begin by adding the students of both sports together.

43 + 37 = 80

But wait! 9 of those students are play both baseball and basketball. To avoid double counting these students, subtract 9 from the total.  

80 – 9 = 71

Now we know 71 students play sports. If there are 100 students, all that's left to do is subtract.

100 – 71 = 29

29 students do not play basketball or baseball.

Example Question #1 : Venn Diagrams

There are  students in a class.   of them take German and  take Latin.  Some students take two languages.  There are  students who take no language whatsoever.  How many students are there who take at least one language?

Possible Answers:

Correct answer:

Explanation:

You could represent this question in the following Venn Diagram:

Venn-lg

We know that the two circles must contain a total of  students.  Now, since there is an extra case of the overlap area when you add together the German and the Latin students, you can say:

This is the simple answer to this question!  You do not need to compute the overlap at all, as you are merely looking for the contents of the two circles.

Example Question #1 : Probability

A jar contains 10 red marbles, 4 white marbles, and 2 blue marbles. Two are drawn in sequence, not replacing after each draw.

Quantity A                                                               

The probability of drawing two red marbles    

Quantity B                        

The probability of drawing exactly one blue marble.

Possible Answers:

The quantities are equal.

The relationship cannot be determined from the information given.

Quantity B is greater.

Quantity A is greater.

Correct answer:

Quantity A is greater.

Explanation:

Note that there are 16 total marbles. A is simply a set of sequential events. On the first, you have 10/16 chances to draw a red. Supposing this red is not replaced, the chance of drawing a second red will be 9/15; therefore, the probability of A is (10/16) * (9/15) = 0.375. Event B is translated into 2 events: Blue + (White or Red) or (White or Red) + Blue. The probabilities of each of these events, added together would be (2/16) * (14/15) + (14/16) * (2/15) = 0.2333333333; therefore, A is more probable.

Example Question #1 : How To Find The Probability Of An Outcome

In a bowl containing 10 marbles, 5 are blue and 5 are pink. If 2 marbles are picked randomly, what is the probability that the 2 marbles will not both be pink?

Possible Answers:

7/8

5/6

2/9

7/9

Correct answer:

7/9

Explanation:

To solve this question, you can solve for the probability of choosing 2 marbles that are pink and subtracting that from 1 to obtain the probability of selecting any variation of marbles that are not both pink.

The probability of picking 2 marbles that are both pink would be the product of the probability of choosing the first pink marble multiplied by the probability of choosing a second pink marble from the remaining marbles in the mix.

This would be 1/2 * 4/9 = 2/9.

To obtain the probability that is asked, simply compute 1 – (2/9) = 7/9.

The probability that the 2 randomly chosen marbles are not both pink is 7/9.

Example Question #1 : Probability

Choose a number at random from 1 to 5.

 

Column A

The probability of choosing an even number

 

Column B

The probability of choosing an odd number

Possible Answers:

Column B is greater

Column A is greater

Cannot be determined

Column A and B are equal

Correct answer:

Column B is greater

Explanation:

There are two even numbers and three odd numbers, so P (even) = 2/5 and P (odd) = 3/5.

Example Question #1 : How To Find The Probability Of An Outcome

Two fair dice are thrown. What is the probability that the outcome will either total 7 or include a 3?

Possible Answers:

2/3

5/12

7/12

1/2

8/9

Correct answer:

5/12

Explanation:

If a die is rolled twice, there are 6 * 6 = 36 possible outcomes. 

Each number is equally probable in a fair die. Thus you only need to count the number of outcomes that fulfill the requirement of adding to 7 or including a 3. These include:

1 6

2 5

3 4

4 3

5 2

6 1

3 1

3 2

3 3

3 5 

3 6

1 3

2 3

5 3

6 3

This is 15 possibilities. Thus the probability is 15/36 = 5/12.

Example Question #1 : Probability

Box A has 10 green balls and 8 black balls. 

Box B has 9 green balls and 5 black balls. 

What is the probability if one ball is drawn from each box that both balls are green?

Possible Answers:

5/14

9/14

19/252

10/49

5/9

Correct answer:

5/14

Explanation:

Note that drawing balls from each box are independent events. Thus their probabilities can be combined with multiplication. 

Probability of drawing green from A:

10/18 = 5/9

Probability of drawing green from B:

9/14

So:

5/9 * 9/14 = 5/14

Example Question #1 : Probability

The probability that events A and/or B will occur is 0.88.

Quantity A: The probability that event A will occur.

Quantity B: 0.44.

Possible Answers:

The two quantities are equal.

Quantity B is greater.

Quantity A is greater.

The relationship cannot be determined from the information given.

Correct answer:

The relationship cannot be determined from the information given.

Explanation:

The only probabilites that we know from this is that P(only A) + P(only B) + P (A and B) = 0.88, and that P(neither) = 0.12. We cannot calculate the probability of P(A) unless we know two of the probabilites that add up to 0.88.

Example Question #1 : Outcomes

a is chosen randomly from the following set:

{3, 11, 18, 22}

b is chosen randomly from the following set:

{ 4, 8, 16, 32, 64, 128}

What is the probability that a + b = 27?

Possible Answers:

0.04

0.5

0.1

0.05

0.03

Correct answer:

0.04

Explanation:

Since any of the first set can be summed with any of the second set, the addition sign in the equation works like a conjunction. As such, there are 4 * 6 = 24 possible combinations of a and b. Only one of these combinations, 11 + 16 = 27, works. Thus the probability is 1/24, or about 0.04.

Example Question #3 : How To Find The Probability Of An Outcome

There are four aces in a standard deck of playing cards. What is the approximate probability of drawing two consecutive aces from a standard deck of 52 playing cards?

Possible Answers:

0.004

0.4

0.5

0.005

0.05

Correct answer:

0.005

Explanation:

Answer: .005
Explanation: The probability of two consecutive draws without replacement from a deck of cards is calculated as the number of possible successes over the number of possible outcomes, multiplied together for each case. Thus, for the first ace, there is a 4/52 probability and for the second there is a 3/51 probability. The probability of drawing both aces without replacement is thus 4/52*3/51, or approximately .005.

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