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# The probability of success in a field event is the ninth part of the

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Intern
Joined: 17 Jun 2013
Posts: 6
The probability of success in a field event is the ninth part of the  [#permalink]

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16 Apr 2018, 05:57
00:00

Difficulty:

75% (hard)

Question Stats:

20% (01:12) correct 80% (00:55) wrong based on 10 sessions

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The probability of success in a field event is the ninth part of the probability of failure. Successive and independent trials are held until success occurs for the first time. In these circumstances, the expected number of failures that should occur until the first success occurs is equal to:

(A) 2
(B) 3
(C) 6
(D) 9
(E) 10

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Prof Marcelo Roseira
Curso FDX
GMAT Prep Online
Brazil
Instagram: @cursofdx
Manager
Joined: 03 Apr 2016
Posts: 89
Location: India
Schools: ISB '19, IIMB EPGP"20
GMAT 1: 580 Q43 V27
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GRE 1: Q160 V151
GPA: 3.99
WE: Design (Consulting)
The probability of success in a field event is the ninth part of the  [#permalink]

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16 Apr 2018, 06:13
1
CursoFDX wrote:
The probability of success in a field event is the ninth part of the probability of failure. Successive and independent trials are held until success occurs for the first time. In these circumstances, the expected number of failures that should occur until the first success occurs is equal to:

(A) 2
(B) 3
(C) 6
(D) 9
(E) 10

From the question stem, we understand that, probability of success P(s) is 1/9 of probability of failure P(f).
P(s) = 1/9 P(f).
Now analyzing from the point of view of options, if the number of trials before first success is 2 then P(s) =1/2 and P(f) = 1/2 (Wrong, Not what question stem mentions)
From option B, P(s) = 1/3, P(f) = 2/3, (Wrong, question stem needs P(s) = 1/9P(f)
from option C, P(s) = 1/6, P(f) = 5/6. (Wrong)
From option D , P(s) = 1/9, P(f) = 8/9 (Wrong)
From option E, P(s) = 1/10, P(f) = 9/10 => P(s)/P(f) = 1/9 => P(s)=1/9 P(f) (Right Answer)
Hence option E is right answer
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Concentration: Marketing, Technology
GMAT 1: 630 Q44 V33
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GPA: 3.25
WE: Marketing (Telecommunications)
Re: The probability of success in a field event is the ninth part of the  [#permalink]

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16 Apr 2018, 06:23
CursoFDX wrote:
The probability of success in a field event is the ninth part of the probability of failure. Successive and independent trials are held until success occurs for the first time. In these circumstances, the expected number of failures that should occur until the first success occurs is equal to:

(A) 2
(B) 3
(C) 6
(D) 9
(E) 10

Let Failure = x, so Success= x/9. Since S+F=1 --> x + x/9 = 1 --> F=9/10 and S=1/10

{9F}+{1S}=10. I got max 9 Failures before 1 Success, why 10 F (E)? The Q asks how many F not trials why 10 and not 9???
Thanks.
Intern
Joined: 17 Jun 2013
Posts: 6
Re: The probability of success in a field event is the ninth part of the  [#permalink]

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19 Apr 2018, 17:55
1
That is the problem. I also marked letter (D). I think that official answer (E) is wrong.

Hero8888 wrote:
CursoFDX wrote:
The probability of success in a field event is the ninth part of the probability of failure. Successive and independent trials are held until success occurs for the first time. In these circumstances, the expected number of failures that should occur until the first success occurs is equal to:

(A) 2
(B) 3
(C) 6
(D) 9
(E) 10

Let Failure = x, so Success= x/9. Since S+F=1 --> x + x/9 = 1 --> F=9/10 and S=1/10

{9F}+{1S}=10. I got max 9 Failures before 1 Success, why 10 F (E)? The Q asks how many F not trials why 10 and not 9???
Thanks.

_________________
Prof Marcelo Roseira
Curso FDX
GMAT Prep Online
Brazil
Instagram: @cursofdx
Re: The probability of success in a field event is the ninth part of the   [#permalink] 19 Apr 2018, 17:55
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