All GMAT Math Resources
Example Questions
Example Question #2 : Calculating Discrete Probability
A bag has 7 blue balls and 3 red balls. 2 balls are to be drawn successively and without replacement. What is the probability that the first ball is red and the second ball is blue?
We first have 7 blue and 3 red out of 10, so P(1st ball is red) = . Now, we have pulled a red ball out of the bag, leaving us with 7 blue and 2 red out of 9 total balls. Then P(2nd ball is blue) = . Put this together, P(1st red AND 2nd blue) = .
Example Question #2 : Calculating Discrete Probability
4 cards are to be dealt successively and without replacement from an ordinary deck of 52 cards. What is the probability of receiving, in order, a spade, a heart, a diamond, and a club?
There are 4 suits, and each suit has 13 cards, so P(spade) = .
Now there are 51 cards left: 12 spades, 13 hearts, 13 diamonds, and 13 clubs, so now P(heart) = .
Once again, there are now 50 cards: 12 spades, 12 hearts, 13 diamonds, and 13 clubs, so now P(diamond) = .
Before our last draw we have 49 cards left: 12 spades, 12 hearts, 12 diamonds, and 13 clubs, so P(club) = .
Putting these four probabilities together gives us our answer:
P(spade AND heart AND diamond AND club) =
Example Question #1 : Discrete Probability
A red die and a white die are rolled. What is the probability of getting a 4 on the red die AND an odd sum of numbers on the two dice?
Let's first look at the two probabilities separately.
There are 36 possible combinations of the two dice: (1, 1), (1, 2), (1, 3),...,(1, 6); (2, 1), (2, 2), (2, 3),...; ....; (6, 1), ..., (6, 6). Getting a 4 on the red die can happen 6 different ways: (4, 1), (4, 2), (4, 3), (4, 4), (4, 5), and (4, 6). So P(4 on red die)
.
Now, getting an odd sum on the dice can happen 18 different ways: (1, 2), (1, 4), (1, 6), (2, 1), (2, 3), (2, 5), (3, 2), (3, 4), (3, 6), (4, 1), (4, 3), (4, 5), (5, 2), (5, 4), (5, 6), (6, 1), (6, 3), and (6, 5). So P(odd sum) =
.
Putting them together, P(4 on red AND odd sum) =
.
Example Question #4 : Discrete Probability
What is the probability of sequentially drawing 2 diamonds from a deck of regular playing cards when the first card is not replaced?
The probability of drawing a diamond first is
The probabilty of drawing a diamond second when the first card is not replaced is .
To determine the probability of 2 INDEPENDENT events, we multiply them.
Example Question #5 : Discrete Probability
Two fair dice are tossed. What is the probability that the dice will add up to a prime number?
The prime numbers that can be rolled are 2, 3, 5, 7, and 11. The rolls that result in one of these numbers, out of a possible rolls, are:
Therefore, there are 15 rolls out of 36 that result in a prime number, making the probability of a prime result
Example Question #4 : Discrete Probability
Two fair six-sided dice are rolled. What is the probability that the sum is either a perfect square or a perfect cube?
There are 36 possible rolls. The only perfect squares that can be rolled are 4 and 9; the only perfect cube that can be rolled is 8. So the roll must be one of the following:
This adds up to 12 rolls out of 36, for a probability of
Example Question #251 : Arithmetic
Three coins are tossed at the same time. One of them is fair; two are loaded so that each comes up heads with probability . In terms of , what is the probability that the outcome will be three heads?
The probability that all three coins will come up heads is the product of the individual probabilities:
Example Question #252 : Arithmetic
One hundred marbles - each one red, yellow, blue, or green - are placed in a box. Forty of the marbles are green, there are twice as many blue marbles as there are red ones, and there are three times as many yellow marbles as red ones. What is the probability that a randomly drawn marble will be yellow?
Let be the number of red marbles. Then there are blue marbles and yellow ones. Since there are 40 green marbles, and 100 marbles total, we can write this equation, simplifying and solving for :
Therefore, there are yellow marbles, and the probability of drawing a yellow marble is
Example Question #13 : Discrete Probability
Which of the following can be done to a standard deck of fifty-two playing cards without affecting the probability that a randomly chosen card will be a club?
Removing the hearts
Removing the red kings
Adding the joker
Removing the fours
Removing the ace of spades
Removing the fours
The probability of drawing a club from a standard deck of cards is , since one-fourth of the cards - thirteen out of fifty-two - are clubs.
If the two red kings are removed, thirteen out of the remaining fifty cards are clubs, and the probability is .
If the thirteen hearts are removed, thirteen out of the remaining thirty-nine cards are clubs, and the probability is .
If the ace of spades is removed, thirteen out of the remaining fifty-one cards are clubs, and the probability is .
If the joker is added, thirteen out of the fifty-three cards are clubs, and the probability is .
If the four fours are removed, this leaves twelve clubs out of forty-eight cards, so the probability is . This is the corect choice.
Example Question #1801 : Gmat Quantitative Reasoning
Four coins are tossed at the same time. Three of them are fair; the fourth is loaded so that it comes up heads with probability . In terms of , what is the probability that the outcome will be three heads and one tail?
Call the fair coins, which come up heads or tails with probability , Coins 1, 2, and 3; call the loaded coin, which comes up heads with probability and tails with probability , Coin 4. We are looking for the probability of one of the following four outcomes:
The probability of heads on Coins 1, 2, and 3 and tails on Coin 4:
The probability of tails on Coin 1 and heads on Coins 2, 3, and 4:
The probability of tails on Coin 2 and heads on Coins 1, 3, and 4:
The probability of tails on Coin 3 and heads on Coins 1, 2, and 4:
Add these: