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# QOTD # 19 For a certain probability experiment, the probabil

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QOTD # 19 For a certain probability experiment, the probabil [#permalink]  12 Sep 2016, 02:02
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For a certain probability experiment, the probability that event A will occur is \frac{1}{2} and the probability that event B will occur is \frac{1}{3}. Which of the following values could be the probability that the event A ∪ B (that is, the event A or B, or both) will occur?

Indicate all such values.

A) \frac{1}{3}

B) \frac{1}{2}

C) \frac{3}{4}

[Reveal] Spoiler: OA
B and C

Practice Questions
Question: 19
Page: 181
[Reveal] Spoiler: OA

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Re: QOTD # 19 For a certain probability experiment, the probabil [#permalink]  12 Sep 2016, 02:10
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Explanation

Since you know that the probability of event A is \frac{1}{2}and the probability of event B is \frac{1}{3} but you are not given any information about the relationship between events A and B, you can compute only the minimum possible value and the maximum possible value of the probability of the event A ∪ B. The probability of A ∪ B is least if B is a subset of A; in that case, the probability of A ∪ B is just the probability of A, or \frac{1}{2} .

The probability of A ∪ B is greatest if A and B do not intersect at all; in that case, the probability of A ∪ B is the sum of the probabilities of A and B, or \frac{1}{2} + \frac{1}{3} = \frac{5}{6} With no further information about A and B, the probability that A or B, or both, will occur could be any number from \frac{1}{2} to \frac{5}{6}.
Of the answer choices given, only \frac{1}{2} and \frac{3}{4} are in this interval.

The correct answer consists of Choices B and C.
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Re: QOTD # 19 For a certain probability experiment, the probabil [#permalink]  11 Feb 2018, 19:13
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Dear Carcass, please could you help me…

Since we don’t know the relationship between A and B my assumption is based on the following

Two extreme cases:

1. If the events A and B are not mutually exclusive, then p(A or B) = p(A) + p(B) - p(A and B).
p(A or B) = ½+1/3-1/6=2/3

2. If events A and B are mutually exclusive, then the probability of A or B is simply:
p(A or B) = p(A) + p(B).
p(A or B) = ½+1/3=5/6
Thus, I assumed correct answer should be just C
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Re: QOTD # 19 For a certain probability experiment, the probabil [#permalink]  12 Feb 2018, 04:07
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Expert's post
This is the correct expression

P(A ∪ B) = P(A) + P(B) - P(B)P( A | B )

P(A) is probability of A happening
P(B) is probability of B happening
P( B | A ) can be said to Probability of B given that A has occured. Now look at the figure below. The area P( A | B ) is minimum when P(A) and P(B) do not overlap at all.

Attachment:

Inkedgeom121211_LI.jpg [ 835.65 KiB | Viewed 147 times ]
.
Intutive explanation of the formula:

P(A ∪ B) = P(A) + P(B) - P(B)P( A | B )

If you look at the figure to find area of P(A ∪ B) we add the area of P(A) and P(B) but if P(A) and P(B) overlap, we would add the overlapping area twice! So we sunbtract P(B)P( A | B ).

Example:

An urn contains 6 red marbles and 4 black marbles. Two marbles are drawn without replacement from the urn. What is the probability that both of the marbles are black?

Let A = the event that the first marble is black; and let B = the event that the second marble is black.

We know the following:

In the beginning, there are 10 marbles in the urn, 4 of which are black. Therefore, P(A) = 4/10.
After the first selection, there are 9 marbles in the urn, 3 of which are black. Therefore, P(B|A) = 3/9.

P(A) P(B|A) = (4/10) * (3/9) = 12/90 = 2/15 = 0.133

P(B)P( A | B ) is not P(B)P(A). The amount of overlapping area cannot be found out unless specified

Please take care of this as this is the very fundamental concept of probability. Hope this helps and feel free to ask more questions.
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Re: QOTD # 19 For a certain probability experiment, the probabil   [#permalink] 12 Feb 2018, 04:07
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