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Probability

by sukhman » Sun Sep 08, 2013 10:36 pm
A carton contains 25 bulbs , 8 of which are defective. What is the probability that if a sample of 4 bulbs is chosen , exactly 2 of them will be defective ?
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by [email protected] » Sun Sep 08, 2013 11:20 pm
Hi sukhman,

What is the source of this question? And where are the answer choices? These numbers are rather large for what the GMAT writers would put into a question of this type, which makes me think that it's not a representative question.

The solution has to take into account the number of defective and non-defective bulbs as well as the total bulbs. In addition, ANY 2 of the 4 bulbs could be defective, so you have to factor in the combination formula.

It would be this: [spoiler](4c2)(8x7x17x16)/(25x24x23x22)[/spoiler]

This math is something the GMAT would NOT ask you to solve.

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