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Probability

This topic has 2 expert replies and 3 member replies
Imsukhi Senior | Next Rank: 100 Posts Default Avatar
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Probability

Post Sat Jun 22, 2013 4:09 am
There are 10 envelopes and 10 letters to go inside them. Each letter is menat for a specified envelope only. What is the probability that exactly 9 of them are in the right envelope?

a)1/10!
b)0
c)1
d)9/10
E) none of above

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Post Sat Jun 22, 2013 10:33 am
snigdha1605 wrote:
Hi Brent,

Wouldn't this problem translate to the probability of getting all of the letters in their correct envelopes which would be 1/10! ?

Total no of ways of placing the letters in the envelopes = 10!
No of ways of placing correct letters within their corresponding envelopes = 1

Regards,
Snigdha
The question asks for the probability that exactly exactly 9 letters are in the right envelope. This is different from asking for the probability that all 10 letters are in the right envelope.

Since it's impossible for exactly 9 letters to be in the right envelope, the probability is zero.

Cheers,
Brent

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snigdha1605 Senior | Next Rank: 100 Posts
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Post Sat Jun 22, 2013 11:26 am
Thanks Brent...Got it now

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Post Sat Jun 22, 2013 10:33 am
snigdha1605 wrote:
Hi Brent,

Wouldn't this problem translate to the probability of getting all of the letters in their correct envelopes which would be 1/10! ?

Total no of ways of placing the letters in the envelopes = 10!
No of ways of placing correct letters within their corresponding envelopes = 1

Regards,
Snigdha
The question asks for the probability that exactly exactly 9 letters are in the right envelope. This is different from asking for the probability that all 10 letters are in the right envelope.

Since it's impossible for exactly 9 letters to be in the right envelope, the probability is zero.

Cheers,
Brent

_________________
Brent Hanneson – Founder of GMATPrepNow.com
Use our video course along with Beat The GMAT's free 60-Day Study Guide

Check out the online reviews of our course
Come see all of our free resources

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snigdha1605 Senior | Next Rank: 100 Posts
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Post Sat Jun 22, 2013 11:26 am
Thanks Brent...Got it now

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Imsukhi Senior | Next Rank: 100 Posts Default Avatar
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Post Sat Jun 22, 2013 9:43 am
Hi Snigdha,

Brent is rite. The probability is zero.

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