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by sumanr84 » Sat Feb 13, 2010 10:04 am
Q. A lot of fifteen items contains five defective items. Two items are drawn at random. What is the probability that the second item drawn will be defective?

OA : [spoiler]1/3[/spoiler]

Pls help me in understanding the approach.
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by ajith » Sat Feb 13, 2010 12:40 pm
sumanr84 wrote:Q. A lot of fifteen items contains five defective items. Two items are drawn at random. What is the probability that the second item drawn will be defective?

OA : [spoiler]1/3[/spoiler]

Pls help me in understanding the approach.
The easy way to tell it is this - we do not know what happened when the first item was drawn - whether it was defective or not so, it can be defective or non defective with a probability of 1/3 (5/15). Since we do not the outcome of the first the probability of the second item drawn is defective is 1/3 itself.

But Given the first one was defective the probability changes
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by vscid » Sat Feb 13, 2010 12:43 pm
Well I think since both are drawn together and since we know nothing about the other, the probability of the 2nd one as well as the first one is 5/15.
The GMAT is indeed adaptable. Whenever I answer RC, it proficiently 'adapts' itself to mark my 'right' answer 'wrong'.
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by komal » Tue Feb 16, 2010 8:32 am
sumanr84 wrote:Q. A lot of fifteen items contains five defective items. Two items are drawn at random. What is the probability that the second item drawn will be defective?

OA : [spoiler]1/3[/spoiler]

Pls help me in understanding the approach.

First item can either be good, or defective. So our possible sets is:

First item: Good, Second item: Defective
OR
First item: Defective, Second item: Defective

The probability is therefore

P = 10/15 * 5/14 + 5/15 * 4/14 = 1/3
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