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# m17, q36

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Manager
Joined: 10 Jul 2009
Posts: 125

Kudos [?]: 187 [0], given: 60

Location: Ukraine, Kyiv

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22 Nov 2009, 04:04
The probability that event A will occur is 20%. If event A occurs, the probability that event B will occur is 30%. What is the probability that event A will occur but event B will not?

1/10
7/55
5/36
7/50
5/24

I agree with OE here, but I do not agree with the solving method.
OE states:
The probability that event A will occur but event B will not = (the probability that event A will occur)*(1 - the probability that event B will occur) =
BUT: the solution is: 1/5*(1-0.3)=1/5*7/10=7/50.

May be I misinterpreted the question stem, but for me the probability that event B WILL occur is 1/5*0.3. Isn't it?
_________________

Never, never, never give up

Kudos [?]: 187 [0], given: 60

Senior Manager
Joined: 31 Aug 2009
Posts: 415

Kudos [?]: 358 [0], given: 20

Location: Sydney, Australia
Re: m17, q36 [#permalink]

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22 Nov 2009, 05:07
I think the OE is correct. The question asks what is the probability that B will NOT occur.

We want P(A) and P(Not B). This translates to:
P(A) * P(Not B) = P(A) * (1-P(B))
P(A) = 20%
P(B) = 30%
1-P(B) = 1-30% = 70%

P(A) * (1-P(B)) = 20%*70% = 1/5 * 7/10 = 7/50

Kudos [?]: 358 [0], given: 20

Intern
Joined: 13 Sep 2011
Posts: 2

Kudos [?]: 5 [0], given: 0

Location: Singapore
Concentration: Finance, Economics
GMAT 1: 720 Q50 V37
GRE 1: 1430 Q790 V640
GPA: 3.1
Re: m17, q36 [#permalink]

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29 Oct 2011, 18:41
I disagree with the explanation.

I agree that the final solution is P(A) * [1-P(B)].
& P(A) = 0.2

However regarding P(B) the question says;
" If event A occurs, the probability that event B will occur is 30%"
therefore P(B/A) = 0.3 and not P(B) = 0.3

In the explanation P(B) has been assumed to be 0.3, this does not look right to me.

I think P(B/A) = P (A and B) / P(A) = 0.3 = [P(A) + P(B) - P(A or B)] / P(A)

Kudos [?]: 5 [0], given: 0

Director
Status: Gonna rock this time!!!
Joined: 22 Jul 2012
Posts: 506

Kudos [?]: 72 [0], given: 562

Location: India
GMAT 1: 640 Q43 V34
GMAT 2: 630 Q47 V29
WE: Information Technology (Computer Software)
Re: m17, q36 [#permalink]

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24 Feb 2013, 03:49
bunuel,

Why on earth is ' If event A occurs' is included in the question. This doesn't seem alright.

Regards,
SAch
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Kudos [?]: 72 [0], given: 562

Re: m17, q36   [#permalink] 24 Feb 2013, 03:49
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# m17, q36

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Moderator: Bunuel

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