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# Event A and Event B are independent. Is the probability that

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CEO
Joined: 21 Jan 2007
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Event A and Event B are independent. Is the probability that [#permalink]

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23 Oct 2007, 08:13
Event A and Event B are independent. Is the probability that both Event A and Event B will happen greater than .3?

(1) P(A) = .25
(2) P(not B) = .71

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Manager
Joined: 18 Jan 2012
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Schools: IIM A '15 (A)
Re: Event A and Event B are independent. Is the probability that [#permalink]

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07 Nov 2012, 18:57
1
KUDOS
What a wonderful question.
Guyz, beware of the old GMAT trick. If something looks too obvious, then one should immediately take a step back. The GMAT is all
about quantitative REASONING. This is why the GMAT is a difficult exam. The TEST IS DESIGNED to trouble test takers who prefer a
"I know the formula by rote, let me plug in some numbers approach". I should know. I am a card carrying member of the "learn By rote" gang.

Whenever the GMAT gives you a "Is "X" less than 1 " or "Is X greater than 2" question, then this is a clear indication that one has to
test boundary conditions. Let's think for a moment. The question stem is phrased in a manner "Is X more than something"...
Why would somebody ask us this question ? The question stem kinda implies that the probability can swing within a range.

If the question stem was phrased as "What is the probability.....", then this would imply a calculation intensive approach.

The maximum probability that an event can occur is 1
The minimum probability that an event can occur is 0

If we apply these concepts to the question, we will realize that the maximum probability in each case will always be less than
0.3
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VP
Joined: 10 Jun 2007
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Re: Challenge - Independent Events [#permalink]

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23 Oct 2007, 08:18
bmwhype2 wrote:
Event A and Event B are independent. Is the probability that both Event A and Event B will happen greater than .3?

1. P(A) = .25
2. P(not B) = .71

C
We want to find P(A)*(P(B)
(1) tells P(A)
(2) tells P(B) = 1 - P(not B)
Both is SUFFICIENT

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CEO
Joined: 21 Jan 2007
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23 Oct 2007, 09:06
I think there is an error in GMATClub's answer. The OA is D.

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Current Student
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23 Oct 2007, 09:21
I get C too

P (A and B) = P(a)*p(b)

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Manager
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23 Oct 2007, 09:25
1
This post was
BOOKMARKED
D is correct

1) P(A) = .25
If P(B) = 1 (greatest value), then this would still be .25 and less than .3. So no matter what the probability of P(B), this would never be greater than .3.
SUFFICIENT

2) P(not B) = .71 or P(B) = .29. Same reasoning found in 1).
SUFFICIENT

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VP
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23 Oct 2007, 09:27
yuefei wrote:
D is correct

1) P(A) = .25
If P(B) = 1 (greatest value), then this would still be .25 and less than .3. So no matter what the probability of P(B), this would never be greater than .3.
SUFFICIENT

2) P(not B) = .71 or P(B) = .29. Same reasoning found in 1).
SUFFICIENT

I agree.
I did it wrong.

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23 Oct 2007, 09:27
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# Event A and Event B are independent. Is the probability that

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