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# There are seven passwords on the list and only one of them

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Manager
Joined: 01 Jan 2008
Posts: 220
Schools: Booth, Stern, Haas
There are seven passwords on the list and only one of them [#permalink]

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22 Oct 2008, 00:11
There are seven passwords on the list and only one of them is valid, in order to unlock the lock box you have three trials. What is the probability that you will unlock it? Any explanations will be appreciated

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VP
Joined: 17 Jun 2008
Posts: 1477

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22 Oct 2008, 00:26
Three possible ways: get password right in the first trial, second trial or third trial.

Hence, probability = 1/7 + 6/7*1/6 + 6/7*5/6*1/5
=1/7 + 1/7 + 1/7
= 3/7
VP
Joined: 30 Jun 2008
Posts: 1007

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22 Oct 2008, 01:43
Probability = no. of fav outcomes / tot no of outcomes

tot no of outcomes = 7 (or 7C1)
fav outcomes = 3 (or 3C1)

Prob = 3/7
_________________

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VP
Joined: 05 Jul 2008
Posts: 1333

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22 Oct 2008, 09:23
another way

total outcomes is 7 c 3 = 35

ways we can pick wrong password is 6 c 3 = 20

will not unlock = 20/35 = 4/7

will unlock = 1-4/7 = 3/7

--== Message from GMAT Club Team ==--

This is not a quality discussion. It has been retired.

If you would like to discuss this question please re-post it in the respective forum. Thank you!

To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.
Re: Probability   [#permalink] 22 Oct 2008, 09:23
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# There are seven passwords on the list and only one of them

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