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PS -probability

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PS -probability [#permalink] New post 14 Mar 2009, 02:45
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Question Stats:

0% (00:00) correct 0% (00:00) wrong based on 0 sessions
request your detailed explanation of attached probability question .
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Re: PS -probability [#permalink] New post 14 Mar 2009, 02:46
there was some problem in uploading this question ...Anyway B is not the right choice ..
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Re: PS -probability [#permalink] New post 14 Mar 2009, 04:00
D

One of possible solutions:

Let's find probability of the case when birthdays don't coincide:

1) the number of options for 1-st student: 365 (no restrictions).
2) the number of options for 2-nd student: 365-1 (excepting 1-st student's birthday)
...
85) the number of options for 85-th student: 365-84 (excepting all 84 students' birthdays)

probability = \frac{365*(365-1)...(365-84)}{365^{85}} = \frac{365!}{(365-85)!*365^{85}}

Answer: 1-probability = 1 - \frac{365!}{280!*365^{85}}
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Re: PS -probability   [#permalink] 14 Mar 2009, 04:00
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