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vipin7um
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A lot of fifteen items contains five defective items. Two 19 Apr 2006, 09:31
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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?



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willget800
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A lot of fifteen items contains five defective items. Two 19 Apr 2006, 09:51
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1/3...

5/15*4/14 + 10/15*5/14
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A lot of fifteen items contains five defective items. Two 19 Apr 2006, 10:08
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(1st defective and 2nd defective) or (1st non defective and 2nd defective)

(5C1*4C1)/15C2 + (10C1*5C1)/15C2

20/105 + 50/105

70/105

2/3
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A lot of fifteen items contains five defective items. Two 19 Apr 2006, 10:32
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(1st defective and 2nd defective) or (1st non defective and 2nd defective)
= 5/15*4/14 + 10/15*5/ 14
= 1/3

willget800 is right.
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Charlie45
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A lot of fifteen items contains five defective items. Two 19 Apr 2006, 11:43
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Quote:
(1st defective and 2nd defective) or (1st non defective and 2nd defective)
= 5/15*4/14 + 10/15*5/ 14
= 1/3

willget800 is right.
Show more


I agree with the above as well.
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Nayan
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A lot of fifteen items contains five defective items. Two 19 Apr 2006, 13:08
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One more for 1/3.

same expln as above
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A lot of fifteen items contains five defective items. Two 19 Apr 2006, 14:36
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1/3 is the correct Answer.

Thanks,
Vipin
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willget800
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A lot of fifteen items contains five defective items. Two 19 Apr 2006, 18:16
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gmatacer.. i think you thought of combinations here by selcting 5 out of 15? i am not sure the logic here.. can you explain.. ...
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vickypaul
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A lot of fifteen items contains five defective items. Two 19 Apr 2006, 19:09
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I'm doing this by using combinations but why am I getting wrong? Ideally, the answer should be the same.


(10C1*5C1/15C2) + 5C2/15C2
= (10*5)/(15*7) + (5*4)/(15*14) .................. STEP
= 10/21 + 2/21
= 12/21 :?

"STEP" should get same answer by willget800's method. Where am I making mistake? please help....

10/15*5/14 + 5/15*4/14 ................. usual method
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A lot of fifteen items contains five defective items. Two 19 Apr 2006, 21:46
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The problem is, I think, that we are concerned about the order, because we want to know if the second is defective. If we have D(defective)N(non-defective), it is considered as different from ND. So combination should not be used.
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sm176811
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A lot of fifteen items contains five defective items. Two 20 Apr 2006, 06:16
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Agree with 1/3

DD + ND (where N= non deffective and D = deffective)

5/15*4/14 +10/15*5/14 =1/3
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ywilfred
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A lot of fifteen items contains five defective items. Two 20 Apr 2006, 19:07
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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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Hi there,
This topic has been closed and archived due to inactivity or violation of community quality standards. No more replies are possible here.
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