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VCE Stuff => VCE Mathematics => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Mathematical Methods CAS => Topic started by: mano91 on August 20, 2012, 06:48:42 pm

Title: probability question
Post by: mano91 on August 20, 2012, 06:48:42 pm

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!

Title: Re: probability question
Post by: BubbleWrapMan on August 20, 2012, 07:13:39 pm

Hrm, shouldn't it be BI RI DI DB DR?

Title: Re: probability question
Post by: mano91 on August 20, 2012, 08:16:23 pm

no because DI is the intersection of the two events, we were asked for D or a vowel

Title: Re: probability question
Post by: BubbleWrapMan on August 20, 2012, 08:22:41 pm

'Or' is inclusive.

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

Title: Re: probability question
Post by: mano91 on August 20, 2012, 08:27:20 pm

ahh, the solution to the question is wrong! which means that the addition rule can be applied correctly

Title: Re: probability question
Post by: mano91 on August 20, 2012, 08:28:33 pm

thanks buddy :)

Title: Re: probability question
Post by: BubbleWrapMan on August 20, 2012, 08:30:22 pm

The reason you subtract the intersection is because you'd count it twice otherwise -- not because it shouldn't be included.