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September 08, 2025, 01:00:20 am
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mano91

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probability question
« on: August 20, 2012, 06:48:42 pm »
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Two letters are selected from the word BIRD.
What is the probability that the second letter is a vowel or that the first letter is a D?

Using a tree diagram, it is obvious to see that the answer is 4/12 = 1/3.  (BI,RI, DI, DR.)
But how come applying the addition rule: Pr (D U Vowel) = pr(d) + pr(vowel) - pr( D and Vowel ) doesn't work?
unless i am applying it wrong... thanks!
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BubbleWrapMan

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Re: probability question
« Reply #1 on: August 20, 2012, 07:13:39 pm »
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Hrm, shouldn't it be BI RI DI DB DR?
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mano91

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Re: probability question
« Reply #2 on: August 20, 2012, 08:16:23 pm »
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no because DI is the intersection of the two events, we were asked for D or a vowel
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BubbleWrapMan

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Re: probability question
« Reply #3 on: August 20, 2012, 08:22:41 pm »
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'Or' is inclusive.

EDIT: I mean wait what. Why don't you include DB?
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mano91

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Re: probability question
« Reply #4 on: August 20, 2012, 08:27:20 pm »
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ahh, the solution to the question is wrong! which means that the addition rule can be applied correctly
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mano91

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Re: probability question
« Reply #5 on: August 20, 2012, 08:28:33 pm »
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thanks buddy :)
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BubbleWrapMan

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Re: probability question
« Reply #6 on: August 20, 2012, 08:30:22 pm »
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The reason you subtract the intersection is because you'd count it twice otherwise -- not because it shouldn't be included.
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