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From a bunch of five keys, three particular keys have to be

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From a bunch of five keys, three particular keys have to be  [#permalink]

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New post 13 Oct 2013, 01:31
4
00:00
A
B
C
D
E

Difficulty:

  25% (medium)

Question Stats:

73% (01:29) correct 27% (02:14) wrong based on 78 sessions

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From a bunch of five keys, three particular keys have to be used to open a combination lock, such that the second key can be used only after the first key and they third key can be used only after the second key. What are the chances of opening the lock in a random selection of three keys?

A. 1/20
B. 1/30
C. 1/60
D. 1/80
E. 1/120

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Re: From a bunch of five keys, three particular keys have to be  [#permalink]

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New post 13 Oct 2013, 02:58
1
Qoofi wrote:
From a bunch of five keys, three particular keys have to be used to open a combination lock, such that the second key can be used only after the first key and they third key can be used only after the second key. What are the chances of opening the lock in a random selection of three keys?

A. 1/20
B. 1/30
C. 1/60
D. 1/80
E. 1/120


P = 1/5*1/4*1/3 = 1/60.

Answer: C.
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Re: From a bunch of five keys, three particular keys have to be  [#permalink]

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New post 12 Jul 2018, 03:43
Probability of choosing 1 key out of 5 keys = 1/5
Now that 1 key is taken we are left with 4 keys.

Probability of choosing 1 key out of 4 keys = 1/4
Now that 1 key is taken we are left with 3 keys.

Probability of choosing 1 key out of 3 keys = 1/3

Overall probability = 1/5 * 1/4 * 1/3 = 1/60

Option C
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Re: From a bunch of five keys, three particular keys have to be   [#permalink] 12 Jul 2018, 03:43
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From a bunch of five keys, three particular keys have to be

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