GMAT Math : Arithmetic

Study concepts, example questions & explanations for GMAT Math

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Example Questions

Example Question #32 : Calculating Discrete Probability

Marianna inserts the joker into a standard deck of 52 cards. By how much has she decreased the probability that a card drawn at random from the deck will be an ace?

Possible Answers:

Correct answer:

Explanation:

The probability that a card, randomly drawn from a standard deck of 52 cards without the joker, is an ace (or any other given rank) is ; if the joker is added, the probability decreases to . The decrease in probability is

Example Question #33 : Calculating Discrete Probability

Pilar inserts the joker into a standard deck of 52 cards. By how much has she decreased the probability that a card drawn at random from the deck will be a diamond?

Possible Answers:

None of the other responses gives the correct answer.

Correct answer:

Explanation:

The probability that a card, randomly drawn from a standard deck of 52 cards without the joker, is a diamond (or any other given suit) is ; if the joker is added, the probability decreases to . The decrease in probability is

.

Example Question #34 : Calculating Discrete Probability

Judy alters a fair six-sided die so that a roll of 1, 2, 3, 4, or 5 are still equally likely, but a roll of 6 is 5% more likely than any one of the other rolls.

By how much has she increased the probability of rolling a 6?

Possible Answers:

Correct answer:

Explanation:

The probability of rolling a 6 with a fair die is .

Since a roll of 6 is 5% more likely than any one of the other rolls, and the other rolls are equally probable, then, theoretically, there should be 105 rolls of 6 for every 100 rolls of 5, 100 rolls of 4, etc. - that is, 105 out of 605, rolls should theoretically be a 6. This makes the probability of rolling a 6 on the altered die

.

Therefore, Judy's alteration has increased the probability of rolling a 6 by

Example Question #1836 : Gmat Quantitative Reasoning

Carla alters a fair six-sided die so that a roll of 1, 2, 3, 4, or 5 are still equally likely, but a roll of 6 is 4% more likely than any one of the other five outcomes.

By how much has she decreased the probability of rolling a 5?

Possible Answers:

Correct answer:

Explanation:

The probability of rolling a 5 with a fair die is .

Since a roll of 6 on the altered die is 4% more likely than any one of the other rolls, and the other rolls are equally probable, then, theoretically, there should be 104 rolls of 6 for every 100 rolls of 5, 100 rolls of 4, etc. Consequently, 100 out of 604 rolls should theoretically result in a 5. This makes the probability of rolling a 5 on the altered die

.

Therefore, Pilar's alteration has decreased the probability of rolling a 5 by

 

Example Question #1837 : Gmat Quantitative Reasoning

Mick alters a fair coin so that the probability of it coming up heads is 4% greater than that of it coming up tails. By how much has he decreased the probability of it coming up tails?

Possible Answers:

Correct answer:

Explanation:

The probablilty of a fair coin coming up tails is 

If the coin is altered as in the problem, then the odds of the coin coming up tails as opposed to heads are 100 to 104; restated, the probability of it coming up tails is 

Mick has decreased the probability of the coin coming up tails by 

 

Example Question #41 : Discrete Probability

This problem set is designed to have a better understanding of probabilties

We throw two regular six sided dice. What is the probabilty of having a pair?

Possible Answers:

Correct answer:

Explanation:

The correct answer is , since there are 2 dice and we are looking for the probabilty than any pair shows up, the first die can then be any number, therefore we assign it a probability of one and the second die must be the same number as the first, and therefore has a probabilty of . Multiplying these two probabilities, we obtain  .

Example Question #1831 : Problem Solving Questions

We throw two regular six sided dice. What is the probabilty of having a pair of one?

Possible Answers:

Correct answer:

Explanation:

In this problem, we are asked for the probabilty that a specific pair shows up. The probability that the number one shows up on a single die is . Here, we want this event 'the 1 shows up' to occur twice. The result is given by calculating  which is .

Remember, the probability of two succesful events is calculated by multiplying their individual probabilities.

Example Question #283 : Arithmetic

We throw two six-sided dice. What is the probabilty that the sum of the values showing up is equal to ?

Possible Answers:

Correct answer:

Explanation:

The best way to solve this problem is to count the possible outcomes for this event. Indeed, a result of five is given by different sums; 1+4, 2+3, 3+2, 1+4. The fact that we count twice each of these sum may seem counterintuitive but it stems from the fact that in probabilty we consider events to occur successively, whereas in reality the dice have been thrown at the same time. But if they were thrown one after the other, we would note that obtaining a 2 after 3 is different that obtaining a 3 after a 2.

We then have 4 possible outcomes for the success of the event 'the sum is 5'. The total number of possible outcomes is given by the multiplication of all the possible values on each die or , which is equal to 36.

The final answer is obtained by dividing the number of possible outcomes by the total number of outcomes, in other words by calculating .

Example Question #41 : Calculating Discrete Probability

We throw two dice. What is the probability that the sum of the two sides showing up is equal to ?

Possible Answers:

Correct answer:

Explanation:

In this problem, the best way to calculate the answer is to count the possible outcomes. We can see that the sum can be obtained with the following sums: 2+6,3+5, 4+4, 5+3, 6+2 .

We have a total of 5 outcomes. As previously discussed, some sums are counted twice because in probabilty we study events as if they were successive events even when they are not. For this same reason, the 4+4 is counted only once, because a dice yielding a 4 followed by a dice yielding a 4  is the same as a dice yielding a 4 followed by a dice yielding a 4. 

There are a total of  possible outcomes and the final answer is .

Example Question #42 : Calculating Discrete Probability

Let's try to solve more complicated problems using the same logic as in the dice problems.

We have a standard -card deck. We draw two cards without replacement. What is the probabilty of drawing a pair?

Possible Answers:

Correct answer:

Explanation:

Here we are asked for the probability of drawing any pair. Therefore, the first card can be any card and we can assign this event a probability of one.

Furthermore, the second card drawn must be the same card to form a pair. Therefore, its probability is .

We use 51 because we already have drawn a card. You will notice that to form a pair there are 3 other cards in the deck that can be drawn, for example if an ace is drawn first, there must be three other aces in the deck.

 is the final answer

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