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GMAT Probability (Data Sufficiency)
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Author:  questioner [ Mon May 14, 2012 5:48 am ]
Post subject:  GMAT Probability (Data Sufficiency)

Each marble in a bag is either black or white. How many white marbles are there in the bag?
(1) The sum of the probability of picking a black marble and the probability of picking a white marble equals 1.
(2) The probability of picking a black marble equals 1.

A. Statement (1) BY ITSELF is sufficient to answer the question, but statement (2) by itself is not.
B. Statement (2) BY ITSELF is sufficient to answer the question, but statement (1) by itself is not.
C. Statements (1) and (2) TAKEN TOGETHER are sufficient to answer the question, even though NEITHER statement BY ITSELF is sufficient.
D. Either statement BY ITSELF is sufficient to answer the question.
E. Statements (1) and (2) TAKEN TOGETHER are NOT sufficient to answer the question, meaning that further information would be needed to answer the question.


(B) The probability equals to the number of favourable outcomes divided by the number of all possible outcomes. Let’s denote the number of white marbles by w and the number of black marbles by b.

Statement (1) means b/(w + b) + w/(w + b) = 1. We see that this equality is true for ANY positive values. Therefore statement (1) by itself is NOT sufficient to answer the question.

Statement (2) means b/(w + b) = 1. So b = w + b. It yields w = 0. We have the definite number of white marbles. Therefore statement (2) by itself is sufficient to answer the question. The correct answer is B.
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If we assume that there is at least one white marble in the bag, then, we can presuppose that if there were, let's say 1 million marbles in the bag, and only one white marble, the probability of picking a black marble would still be approximated to 1. Just a thought.

Author:  Gennadiy [ Mon May 14, 2012 5:51 am ]
Post subject:  Re: GMAT Probability (Data Sufficiency)

If it is not stated "approximately", "which is the closest", etc. Then the answer is NOT an approximate value.

In real life, sometimes the probability 1/1000001 is as good as 0. But sometimes it is big, for example if we have 100 mln trials and the outcome, which probability is 1/1000001, is highly remarkable (devastating for example).

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