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Math Expert V
Joined: 02 Sep 2009
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For a certain probability experiment, the probability that event A wil  [#permalink]

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Difficulty:   65% (hard)

Question Stats: 9% (01:17) correct 91% (02:11) wrong based on 23 sessions

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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?

I. $$\frac{1}{3}$$

II. $$\frac{1}{2}$$

III. $$\frac{3}{4}$$

A. I only
B. II only
C. III only
D. II and III only
E. I, II and III

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Re: For a certain probability experiment, the probability that event A wil  [#permalink]

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The minimum value of A∪B is when B is subset of A. Hence minimum value of A ∪ B is 1/2.
The maximum value of A∪B is when A and B are two disjoint events, then A ∪ B is 1/2+1/3=5/6

Hence 0.5 ≤ A ∪ B ≤ 0.83
Only 1/2 and 3/4 lies in the interval. IMO D
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Re: For a certain probability experiment, the probability that event A wil  [#permalink]

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P A = 1/2 =0.5
P B = 1/3 ; 0.33
so P A or P B ; 1/2+1/3 ;5/6 ; .83
range is 0.5 to 0.83
so answer option 1/2 & 3/4 since it will cover all the aforementioned ranges
IMO D

Bunuel wrote:
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?

I. $$\frac{1}{3}$$

II. $$\frac{1}{2}$$

III. $$\frac{3}{4}$$

A. I only
B. II only
C. III only
D. II and III only
E. I, II and III

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CrackVerbal Quant Expert B
Joined: 23 Apr 2019
Posts: 15
Re: For a certain probability experiment, the probability that event A wil  [#permalink]

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1
Sometimes the way the question stem is framed gives you a clue on how you need to proceed. The question says, 'which of the following COULD BE TRUE'. This means that we can have multiple possibilities, so instead of trying to get a definite answer, it would be easier to find the maximum and minimum possibilities and get a range.

This is a tricky question where you need to be very careful about making assumptions. Keep in mind that we have not been given any information on the relationship between the two events A and B. Since we have not been given the relationship between the events, the only thing that we can compute here are the maximum and the minimum possibilities.

Now while solving probability questions using the word 'OR', the best way to solve will be to represent the probabilities as a Venn Diagram.

There will be two scenarios here:

1. Mutually Exclusive Events : There will be no intersection between the two circles of the Venn. So the total probability will be the the addition of the two individual probabilities. This will give you the maximum probability that the two events will occur. So the max value here will be 1/3 + 1/2 = 5/6.

2. Mutually Inclusive Events : There will be an intersection between the two circles of the Venn. If the first circle represents the probability of A, the second represents the probability of B, then the Probability (A or B) = P(A) + P(B) - P(A and B). The minimum probability here will be when the B is a subset of A (since A is greater). So P(A ∪ B) is just the probability of A i.e. 1/2.

Since the min value is 1/2 and the max value is 5/6, any value in between the two (inclusive of the two) will be possible values. So II and III will work.

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Re: For a certain probability experiment, the probability that event A wil  [#permalink]

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Bunuel wrote:
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?

I. $$\frac{1}{3}$$

II. $$\frac{1}{2}$$

III. $$\frac{3}{4}$$

A. I only
B. II only
C. III only
D. II and III only
E. I, II and III

The smallest value of P(A U B) is ½ (occurring when B is a subset of A). The largest value of P(A U B) is ½ + ⅓ = ⅚ (occurring when A and B are disjoint). Therefore, ½ ≤ P(A U B) ≤ ⅚.

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If you find one of my posts helpful, please take a moment to click on the "Kudos" button. Re: For a certain probability experiment, the probability that event A wil   [#permalink] 27 May 2019, 05:24
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