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

Author Message
Manager
Joined: 01 Jan 2008
Posts: 222

Kudos [?]: 61 [0], given: 2

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
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

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

Kudos [?]: 61 [0], given: 2

SVP
Joined: 17 Jun 2008
Posts: 1540

Kudos [?]: 278 [0], given: 0

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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

Kudos [?]: 278 [0], given: 0

VP
Joined: 30 Jun 2008
Posts: 1033

Kudos [?]: 688 [0], given: 1

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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
_________________

"You have to find it. No one else can find it for you." - Bjorn Borg

Kudos [?]: 688 [0], given: 1

VP
Joined: 05 Jul 2008
Posts: 1403

Kudos [?]: 423 [0], given: 1

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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

Kudos [?]: 423 [0], given: 1

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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