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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: 227
Schools: Booth, Stern, Haas
Followers: 1

Kudos [?]: 47 [0], given: 2

There are seven passwords on the list and only one of them [#permalink]  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
Joined: 17 Jun 2008
Posts: 1573
Followers: 12

Kudos [?]: 201 [0], given: 0

Re: Probability [#permalink]  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
Joined: 30 Jun 2008
Posts: 1048
Followers: 11

Kudos [?]: 340 [0], given: 1

Re: Probability [#permalink]  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
Joined: 05 Jul 2008
Posts: 1433
Followers: 35

Kudos [?]: 251 [0], given: 1

Re: Probability [#permalink]  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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