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

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Manager
Manager
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Joined: 01 Jan 2008
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Schools: Booth, Stern, Haas
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There are seven passwords on the list and only one of them [#permalink] New post 21 Oct 2008, 23:11
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
SVP
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Re: Probability [#permalink] New post 21 Oct 2008, 23:26
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
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Re: Probability [#permalink] New post 22 Oct 2008, 00:43
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
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Re: Probability [#permalink] New post 22 Oct 2008, 08:23
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
Re: Probability   [#permalink] 22 Oct 2008, 08:23
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There are seven passwords on the list and only one of them

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