GMAT Math : GMAT Quantitative Reasoning

Study concepts, example questions & explanations for GMAT Math

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Example Questions

Example Question #64 : Arithmetic

A marble is selected at random from a box of red, yellow, and blue marbles. What is the probability that the marble is yellow?

Statement 1: Forty of the marbles are not red.

Statement 2: Sixty of the marbles are not blue.

Possible Answers:

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are insufficient to answer the question.

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

EITHER statement ALONE is sufficient to answer the question.

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

Correct answer:

BOTH statements TOGETHER are insufficient to answer the question.

Explanation:

We need to know the number of marbles that are yellow, and the number of marbles there are total. If we let  be the number of red, blue, and yellow marbles, respectively, the two statements say that  and 

These two statements together do not give us enough information. For example, these two situations both fit the conditions given:

Situation 1: , making the probability of drawing a yellow marble 

Situation 2: , making the probability of drawing a yellow marble 

Example Question #3161 : Gmat Quantitative Reasoning

One card from another deck is added to a standard deck of fifty-two cards. The cards are shuffled and one card is removed.

A card is then drawn at random. What is the probability that that card is an ace?

Statement 1: The card that was added was a spade.

Statetment 2: The card that was removed was a jack.

Possible Answers:

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

BOTH statements TOGETHER are insufficient to answer the question.

EITHER statement ALONE is sufficient to answer the question.

Correct answer:

BOTH statements TOGETHER are insufficient to answer the question.

Explanation:

You need to know two things to answer this question - the rank of the added card, and the rank of the removed card. The second statement is useful but not sufficient; the first is irrelevant to the question.

Example Question #3162 : Gmat Quantitative Reasoning

Two fair dice are rolled. What is the probability that each die (not the sum of the dice) will come up a composite number?

Possible Answers:

Correct answer:

Explanation:

There are two composite numbers on each die, 4 and 6. On one die, the probability of rolling a 4 or a 6 is  ; the probabililty of rolling two composite numbers is .

Example Question #3163 : Gmat Quantitative Reasoning

Four coins - a fair penny, a loaded penny, a fair nickel, and a loaded nickel - are tossed. What is the probability that all four will come up heads?

Statement 1: Yesterday, out of 100 tosses, the loaded penny came up heads 70 times.

Statement 2: Yesterday, out of 100 tosses, the loaded nickel came up heads 40 times.

Possible Answers:

EITHER statement ALONE is sufficient to answer the question.

BOTH statements TOGETHER are insufficient to answer the question.

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

Correct answer:

BOTH statements TOGETHER are insufficient to answer the question.

Explanation:

While experiments such as repeated tossings can give an idea of the probability that a coin will come up heads or tails, they do not provide a definitive answer, so neither statement is helpful here.

Example Question #17 : Discrete Probability

Two dice are thrown; one is fair, one is loaded, but each has the usual numbers 1-6 on its faces. What is the probability of the outcome being a sum of 2?

Statement 1: The loaded die will come up 1 with probability .

Statement 2: The loaded die will come up any of the numbers 2, 3, 4, 5, or 6 with probability  each.

Possible Answers:

EITHER statement ALONE is sufficient to answer the question.

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are insufficient to answer the question. 

Correct answer:

EITHER statement ALONE is sufficient to answer the question.

Explanation:

The two statements are actually equivalent; if the loaded die comes up any of 2, 3, 4, 5, or 6 with probability , then it will come up 1 with probability , which is exactly the same assertion as Statement 1 makes.

A sum of 2 can only come up one way: a pair of ones. If you know either statement, then you can calculate the probability of tossing a 2 by multiplying , the probability of a die coming up a 1 on the fair die, by , the probability of a 1 coming up on the loaded die.

Example Question #68 : Arithmetic

Two dice are thrown; one is fair, one is loaded, but each has the usual numbers 1-6 on its faces. What is the probability of the outcome being a sum of 12?

Statement 1: The loaded die will come up 6 with probability .

Statement 2: The loaded die will come up 5 with probability .

Possible Answers:

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

EITHER statement ALONE is sufficient to answer the question.

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are insufficient to answer the question.

Correct answer:

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

Explanation:

The only way the sum of the two dice can come up 12 is a pair of sixes, so the only probabilities that count are those of each die coming up 6; Statement 2 is therefore irrelevant to the problem. If you are given Statement 1, however, then you can calculate the probability of tossing a 12 by multiplying:

Example Question #71 : Arithmetic

One hundred marbles, 75 red and 25 white, are distributed equally between two boxes, denoted Box 1 and Box 2. Box 1 contains 30 red marbles. 

One box is chosen at random, and one marble is chosen at random from that box. What is the probability that the marble chosen will be white?

Possible Answers:

Correct answer:

Explanation:

Box 1 will contain  white marbles; Box 2 will contain  white marbles.

The probability of choosing Box 1 is , and the probability of choosing a white marble from this box is , making the probability of choosing Box 1, then a white marble, .

 

The probability of choosing Box 2 is , and the probability of choosing a white marble from this box is , making the probability of choosing Box 2, then a white marble, .

 

The overall probability of choosing a white marble is the sum of these probabilities:

Example Question #21 : Dsq: Calculating Discrete Probability

You are given two boxes, each with an equal number of marbles. All of the marbles are red or white. Are you more likely to draw a red marble at random from Box 1 or from Box 2?

Statement 1: Two-thirds of all the marbles are red. 

Statement 2: Half the marbles in Box 2 are red.

Possible Answers:

EITHER statement ALONE is sufficient to answer the question.

BOTH statements TOGETHER are insufficient to answer the question.

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

Correct answer:

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

Explanation:

Since both boxes contain the same number of marbles, the one with the most red marbles offers us the greater probability of drawing a red marble. For simplicity's sake, assume that each box contains thirty marbles - these arguments will be valid whatever the number.

From Statement 1, it follows that two-thirds of sixty, or forty, marbles total are red, but either box can contain more than the other.

From Statement 2, it follows that fifteen marbles in Box 2 are red, but no information about Box 1 follows.

From both statements, however, it follows that there are fifteen red marbles in Box 2 and twenty-five red marbles in Box 1, so Box 1 has more red marbles and offers the greater chance of drawing a red marble.

Example Question #22 : Dsq: Calculating Discrete Probability

You are given two boxes of marbles, all either red or white. Are you more likely to draw a red marble at random from Box 1 or from Box 2?

Statement 1: Each box contains the same number of red marbles.

Statement 2: Box 2 contains twice as many white marbles as Box 1.

Possible Answers:

BOTH statements TOGETHER are insufficient to answer the question. 

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

EITHER statement ALONE is sufficient to answer the question.

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

Correct answer:

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

Explanation:

If only Statement 1 is assumed, there is no way of knowing which box offers the highest probability of drawing a red marble, since there is no way to tell which box has the higher portion of its marbles red without knowing how their numbers of white marbles or their total numbers of marbles compare. A smiliar argument holds for Statement 2 alone.

If both statements are known, however, each box has the same number of red marbles, and Box 2 contains more marbles overall - this makes Box 1 the box with the highest portion of its marbles red, and the box that offers the greater chance of drawing a red marble in a random draw.

Example Question #21 : Dsq: Calculating Discrete Probability

Two dice are thrown and their sum is noted. One is fair and one is loaded but both have the usual numbers 1-6 on their faces. Which, if either, is greater - the probability of tossing a 3 or the probability of tossing an 11?

Statement 1: The probability of tossing a 2 on the loaded die is 0.2.

Statement 2: The probability of tossing a 5 on the loaded die is 0.3.

Possible Answers:

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are insufficient to answer the question.

EITHER statement ALONE is sufficient to answer the question.

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

Correct answer:

BOTH statements TOGETHER are insufficient to answer the question.

Explanation:

A 3 can be rolled only by tossing a 1-2, but the loaded die can be either the 1 or the 2; similarly, an 11 can be rolled only by tossing a 5-6, but the loaded die can be either the 5 or the 6. We know only the probability of the loaded die coming up 2 and that of it coming up 5; we do not have this information for 1 or 6. We explore two cases and compare the probabilities of rolling a 3 and rolling an 11.

Case 1: Suppose the loaded die will come up 1 with probability 0.1 and 6 with probability 0.3. The probabilities of rolling a 3 or a 11 are, respectively:

and

making 11 the more likely outcome.

Case 2: Suppose the loaded die will come up 1 with probability 0.3 and 6 with probability 0.1. The probabilities of rolling a 3 or a 11 are, respectively:

and 

making 3 the more likely outcome.

Therefore, the two statements together provide insufficient information to answer the question.

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