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# Victor Moses was trying to open a 12 digit lock set by his friend.He k

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Manager
Joined: 15 Dec 2015
Posts: 115
GMAT 1: 660 Q46 V35
GPA: 4
WE: Information Technology (Computer Software)
Victor Moses was trying to open a 12 digit lock set by his friend.He k  [#permalink]

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31 Jul 2017, 11:13
1
2
00:00

Difficulty:

55% (hard)

Question Stats:

53% (02:03) correct 47% (02:30) wrong based on 35 sessions

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Victor Moses was trying to open a 12 digit lock set by his friend.He knew the first 8 digits of the code.Also, he knew that the last 4 digits were 4,5,7,8 but not the order in which they should be entered at last 4 places.He dials a number at random keeping all the digits he knew in mind.What is the probability that the lock gets open on the third attempt?

A. 1/24
B. 1/8
C. 1/9
D. 4/35
E. 4/24
Intern
Joined: 07 Jun 2017
Posts: 10
Re: Victor Moses was trying to open a 12 digit lock set by his friend.He k  [#permalink]

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31 Jul 2017, 11:34
1
3
If it has to be correct at the third attempt, it has to be Wrong, Wrong, Correct in the first three trials. The total number of combinations is 4! = 4*3*2*1 = 24.
For the first wrong attempt, probability is 23/24. (23 of the combinations are wrong and only 1 right)
For the second wrong attempt, probability is 22/23.(22 of the combinations are wrong and only 1 right)
For the third correct attempt, probability is 1/22. (only 1 is right in the remaining 22 combinations)
Therefore, the answer is 23/24 * 22/23 * 1/22 = 1/24. Therefore A.
You should check out more on this site or sites like khan academy which is a huge online learning site.
Manager
Joined: 15 Dec 2015
Posts: 115
GMAT 1: 660 Q46 V35
GPA: 4
WE: Information Technology (Computer Software)
Re: Victor Moses was trying to open a 12 digit lock set by his friend.He k  [#permalink]

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31 Jul 2017, 12:24
DH99 wrote:
Victor Moses was trying to open a 12 digit lock set by his friend.He knew the first 8 digits of the code.Also, he knew that the last 4 digits were 4,5,7,8 but not the order in which they should be entered at last 4 places.He dials a number at random keeping all the digits he knew in mind.What is the probability that the lock gets open on the third attempt?

A. 1/24
B. 1/8
C. 1/9
D. 4/35
E. 4/24

Third attempt/4th attempt/last attempt or any other attempt probability will always be: p(E)= 1/4!
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Re: Victor Moses was trying to open a 12 digit lock set by his friend.He k   [#permalink] 31 Jul 2017, 12:24
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