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# A box contains 10 light bulbs, fewer than half of which are

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Joined: 08 Nov 2008
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A box contains 10 light bulbs, fewer than half of which are [#permalink]

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06 Jun 2009, 02:35
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A box contains 10 light bulbs, fewer than half of which are defective. Two bulbs are to be drawn simultaneously from the box. If n of the bulbs in box are defective, what is the value of n?
(1) The probability that the two bulbs to be drawn will be defective is 1/15.
(2) The probability that one of the bulbs to be drawn will be defective and the other will not be defective is 7/15.
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07 Jun 2009, 02:37
tenaman10 wrote:
A box contains 10 light bulbs, fewer than half of which are defective. Two bulbs are to be drawn simultaneously from the box. If n of the bulbs in box are defective, what is the value of n?
(1) The probability that the two bulbs to be drawn will be defective is 1/15.
(2) The probability that one of the bulbs to be drawn will be defective and the other will not be defective is 7/15.

D

1. (n/10)*(n-1)/9=1/15 ->n(n-1)=6, n=3 suff
2. 2.(n/10)*(10-n)/9=7/15 -> n(10-n)=21 , n=3 suff
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08 Jun 2009, 09:26
1) n/10*(n-1)/9 = 1/15 => n=3 (suf)
2) n/10* (10-n)/9 + (10-n)/10*n/9=7/15 => n=3 (suf)
D
Re: Prob DS   [#permalink] 08 Jun 2009, 09:26
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