GMAT Math : Data-Sufficiency Questions

Study concepts, example questions & explanations for GMAT Math

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Example Questions

Example Question #62 : Dsq: Calculating Discrete Probability

Twelve indepent trials are run, testing the reliability of a new engine part. What is the probability that the part will fail less than half of the time?

1) The probability that the part will fail three times in six trials is 

2) The probability of the part not failing is 

Possible Answers:

Together the two statements are sufficient.

Neither of the statements, separate or together, is sufficient.

Statement 2 alone is sufficient.

Statement 1 alone is sufficient.

Either of the statements is sufficient.

Correct answer:

Either of the statements is sufficient.

Explanation:

Since this probability question deals with successes and failures, it regards binomial distributions:

Where   is the number of trials,  is the number of 'successes,'  is the probability of exactly  successes in  trials, and  is the probability of a success occuring.

Statement 1 allows for a calculation of  to be used in the probability calculation for the problem.

Statement 2 outright gives a  to calculate the probability asked of in the problem.

Example Question #62 : Discrete Probability

The problem of an engine part performing succesfully is . A researcher performs a series of tests. Given  trials, P is the probability of the part succeeding at least once. Is  ?

1) 

1) 

Possible Answers:

Statement 2 alone is sufficient.

Either of the statements is sufficient.

Neither of the statements, separate or together, is sufficient.

Statement 1 alone is sufficient.

Together the two statements are sufficient.

Correct answer:

Statement 1 alone is sufficient.

Explanation:

Since this probability question deals with successes and failures, it regards binomial distributions:

Where   is the number of trials,  is the number of 'successes,'  is the probability of exactly  successes in  trials, and  is the probability of a success occuring.

The probability of at least one success is the complement of exactly zero successes. Since we're looking at 3 trials, we can find the probability of zero successes to be  and thus the complement, our .

For fewer than three trials, the probability will always be below , but for three or more, it depends on exactly how many trials there are, since the probability increases with increasing number of trials.

Example Question #116 : Arithmetic

 students are being seated randomly in a row of  chairs. What is the probability, that one of the students, Harold, will be seated in the chair second from the left?

1) There are  students.

2) The probability of Maude, another student, sitting on the leftmost seat, while Dennis, another student, has the rightmost seat is .

Possible Answers:

Neither of the statements, separate or together, is sufficient.

Statement 2 alone is sufficient.

Together, the two statements are sufficient.

Statement 1 alone is sufficient.

Either of the statements is sufficient.

Correct answer:

Either of the statements is sufficient.

Explanation:

Either of these statements can enable finding the probability of Harold sitting in a given seat, i.e. .

Statement 1 outright provides it, while Statement 2 can lead towards it with the relation 

Example Question #71 : Dsq: Calculating Discrete Probability

A history class has recently finished their second test. A quarter of the class passed both tests. What percentage of those that passed the first test also passed the second?

1) A third of the class passed the first test.

2) Half of the class passed the second test.

Possible Answers:

Statement 1 alone is sufficient.

Either of the statements is sufficient.

Together, the two statements are sufficient.

Neither of the statements, separate or together, is sufficient.

Statement 2 alone is sufficient.

Correct answer:

Statement 1 alone is sufficient.

Explanation:

This is a conditional probability problem, which follows the form:

In this case  is the probability of passing the first test,  is the probability of passing the second test,  is then the probability of passing the second test, having passed the first, and  is the probability of having passed both. The probability or percentage of students that passed the first test is necessary information.

Example Question #1101 : Data Sufficiency Questions

A card is removed from a standard deck of fifty-two cards, and one card is then chosen at random from the altered deck. Has the probability of the drawn card being red increased or decreased?

Statement 1: The removed card is a spade.

Statement 2: The card removed is a jack.

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.

BOTH statements TOGETHER are insufficient 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.

Correct answer:

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

Explanation:

The probability of drawing a red card from a standard deck is .

If a red card is removed from a standard deck of fifty-two cards, and one card is chosen at random, the probability of it being red is . If a black card is removed, the probability is . Therefore, to answer the question, the color of the card must be known; the rank is irrelevant to the question. Statement 1 - but not Statement 2 - provides sufficient information.

Example Question #121 : Arithmetic

A card is removed from a standard deck of fifty-two cards and replaced by the joker, and one card is then chosen at random from the altered deck. Has the probability of the drawn card being black increased, decreased, or stayed the same?

Statement 1: The replaced card is not a heart.

Statement 2: The replaced card is not a club.

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 insufficient to answer the question. 

Explanation:

The probability of drawing a black card from a standard, unaltered deck is .

If a red card is removed from a standard deck of fifty-two cards and replaced by the joker, and one card is chosen at random, all 26 black cards remain in a deck of 52, so the probability of the chosen card being black is still . If a black card is removed and replaced, there are 25 black cards out of 52 left, so the probability is . Therefore, to answer the question, the color of the card must be known. However, the two statements together, if assumed true, leave two possibilities open - the card could be a diamond (red) or a spade (black). The two statements together provide insufficient information.

Example Question #1107 : Data Sufficiency Questions

A card is removed from a standard deck of fifty-two cards and replaced by a card from another deck with the same backing. One card is then chosen at random from the first (altered) deck. Has the probability of the drawn card being black increased, decreased, or stayed the same?

Statement 1: The removed card and its replacement are both jacks.

Statement 2: The removed card and its replacement are of different suits.

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.

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.

Correct answer:

BOTH statements TOGETHER are insufficient to answer the question. 

Explanation:

Assume both statements to be true. The probability of drawing a black card from a standard, unaltered deck is . We examine two cases.

 

Case 1: The jack of diamonds from the first deck was replaced by the jack of clubs from the other deck. 

Then there are now 27 black cards in a deck of 52. The probability of drawing a black card at random from this deck has increased from  to .

 

Case 2: The jack of clubs from the first deck was replaced by the jack of diamonds  from the other deck. 

Then there are now 25 black cards in a deck of 52. The probability of drawing a black card at random from this deck has decreased from  to .

 

In both scenarios, the conditions of both statements were met; the removed card and its replacement were jacks of different suits. But in one scenario, the probability of drawing a black card increased, and in the other, it decreased. The two statements together provide insufficient information.

 

Example Question #1108 : Data Sufficiency Questions

A card has been removed from a standard deck of fifty-two cards and replaced by the joker. What is the color of the removed card?

Statement 1: The probability of drawing a red card from the altered deck is the same as that of drawing a red card from an unaltered deck.

Statement 2: The probability of drawing a black card from the altered deck is less than that of drawing a black card from an unaltered deck.

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.

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.

Correct answer:

EITHER statement ALONE is sufficient to answer the question.

Explanation:

The replacement of one card by the joker in a deck of 52 decreases the number of cards of the color of the removed card by 1, leaves as is the number of cards of the other color, and leaves as is the total number of cards. Therefore, the probability that a randomly drawn card will be of the color of the removed card will be reduced. and the probability that it will be of the other color will be the same. 

From Statement 1 alone, since the probability of drawing a red card from the altered deck is the same as that of drawing a red card from an unaltered deck, it follows that the removed card is black. From Statement 2 alone, since the probability of drawing a black card from the altered deck has been reduced, it follows again that the removed card is black. 

Example Question #72 : Discrete Probability

A card is removed from a standard deck of fifty-two cards and replaced by a card from another deck with the same backing. One card is then chosen at random from the first (altered) deck. Has the probability of the drawn card being red increased, decreased, or stayed the same?

Statement 1: The removed card and its replacement are both hearts.

Statement 2: Neither the removed card nor its replacement is a spade.

Possible Answers:

EITHER statement ALONE is 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.

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.

Correct answer:

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

Explanation:

Assume Statement 1 alone. There were 26 red cards out of 52 total before the switch, and, since the replacement card was of the same color as the removed card, there were 26 red cards out of 52 after the switch. The probability of drawing a red card stayed the same.

Assume Statement 2 alone. Again, there were 26 red cards out of 52 total before the switch. However, Statement 2 leaves open the possibility of both cards having the same color or different colors, since both black (clubs) and red (hearts and diamonds) cards could have been removed or added. If both cards have the same color, then as in Statement 1, the probability stays the same. But if, for example, the removed card is a club and the added card is a diamond, there are now 27 red cards out of 52, and the probability of drawing a red card has increased to . This makes Statement 2 alone inconclusive.

Example Question #1101 : Data Sufficiency Questions

A card from one deck is added to a second deck with the same backing. One card is then chosen at random from the second (altered) deck. Has the probability of the drawn card being an ace increased, decreased, or stayed the same?

Statement 1: The added card is a face card (jack, king, queen).

Statement 2: The added card is the jack of spades.

Possible Answers:

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. 

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.

Correct answer:

EITHER statement ALONE is sufficient to answer the question.

Explanation:

The probability of drawing an ace (one of thirteen ranks) from an unaltered deck is . From either statement alone, it can be determined that the altered deck has 53 cards, 4 of which are aces; this makes the probability of drawing an ace from this deck . The probability has decreased.

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