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# One of 5 pens is faulty. They are tested one at a time, what

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Senior Manager
Joined: 05 Jun 2008
Posts: 304
One of 5 pens is faulty. They are tested one at a time, what [#permalink]

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15 Oct 2008, 04:58
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One of 5 pens is faulty. They are tested one at a time, what is the probability that the faulty pen is :
(a) 1th first one tested (b) the 2nd one testedt
(b) the rd one tested (d) the last one tested
SVP
Joined: 29 Aug 2007
Posts: 2467

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15 Oct 2008, 13:32
vivektripathi wrote:
One of 5 pens is faulty. They are tested one at a time, what is the probability that the faulty pen is :

(a) 1th first one tested (b) the 2nd one testedt
(b) the 3rd one tested (d) the last one tested

a) 1/5
b) 1/4
c) 1/3
d) 1

Edit: I supposed no/without repleshment.
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Senior Manager
Joined: 31 Jul 2008
Posts: 289

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15 Oct 2008, 15:01
well :

i get 1/5 for all of them
Manager
Joined: 12 Feb 2008
Posts: 177

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16 Oct 2008, 07:18
stallone wrote:
well :

i get 1/5 for all of them

every time you pick one, the # of total decreases, since no replacement.
so when you pick the second one to be defective there is 1 defective out of 4 total.
when you pick the 3rd to be defective, you have already picked two good one before, so the total should be 3, and 1 of these 3 is defective.
i hope this helps.
Senior Manager
Joined: 05 Jun 2008
Posts: 304

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17 Oct 2008, 00:25
vivektripathi wrote:
One of 5 pens is faulty. They are tested one at a time, what is the probability that the faulty pen is :
(a) 1th first one tested (b) the 2nd one testedt
(b) the rd one tested (d) the last one tested

I am not sure but stem says that there are five pens places I I I I I
1 2 3 4 5
and if pick 1st or 2nd or third or last and test it the probability will remain same i.e. 1/5.
Re: PSP001   [#permalink] 17 Oct 2008, 00:25
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