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If I know the probability of A and the probability of B how can I calculate the probability of "at least one of them" occurring?

I was thinking that this is P(A or B) = P(A) + P(B) - P(AB). Is this correct?

If it is, then how can I solve the following problem taken from 'DeGroot - Probability and Statistics':

If 50 percent of families in a certain city subscribe to the morning newspaper, 65 percent of the families subscribe to the afternoon newspaper, and 85 percent of the families subscribe to at least one of the two newspapers, what proportion of the families subscribe to both newspapers?

Here the question is P(morning)=.5, P(afternoon)=.65 P(morning OR afternoon)=.85

P(morning OR afternoon) = .5 + .65 - .3 = .85

P(morning AND afternoon) = P(morning) + P(afternoon) - P(morning OR afternoon)

So the answer to the question is .3

Is my reasoning correct?

EDIT.

If this reasoning is correct:

How can I calculate the following:

If the probability that student A will fail a certain statistics examination is 0.5, the probability that student B will fail the examination is 0.2, and the probability that both student A and student B will fail the examination is 0.1,what is the probability that exactly one of the two students will fail the examination?

So this questions highlights the difference between !at least one of them! !or exactly one of them!

I understand that at least one of them = P(A or B)

but how can I work out the probability of exactly one of them?

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Somebody knows an algorith to calculate the OR for n probabilistic values? Thanks in advance. Regards, Daniel. –  Daniel Mejia Nov 8 '12 at 23:53
    
You need to specify the correlation between the $n$ probabilistic values - at least whether they are independent or not. –  dexter04 Nov 9 '12 at 0:25

4 Answers 4

up vote 3 down vote accepted

You are correct.

To expand a little: if $A$ and $B$ are any two events then

$$P(A\textrm{ or }B) = P(A) + P(B) - P(A\textrm{ and }B)$$

or, written in more set-theoretical language,

$$P(A\cup B) = P(A) + P(B) - P(A\cap B)$$

In the example you've given you have $A=$ "subscribes to a morning paper" and $B=$ "subscribes to an afternoon paper." You are given $P(A)$, $P(B)$ and $P(A\cup B)$ and you need to work out $P(A\cap B)$ which you can do by rearranging the formula above, to find that $P(A\cap B) = 0.3$, as you have already worked out.

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thanks for your answer! please have a look at my edited question, your help highlighted a further issue.. –  Dbr Oct 14 '11 at 11:10
1  
"Exactly one of A and B" means "Either A or B, but not both" which you can calculate as P(A or B) - P(A and B). –  Chris Taylor Oct 14 '11 at 11:13
    
thank you, this clarifies everything. Just one last quick thing: Where can I learn the mathematical notation you use on this forum? For some questions I find it quite hard to understand them, is there a tutorial? –  Dbr Oct 14 '11 at 11:15
    
Are you asking about the notation itself, or the method of displaying the notation? To write the notation we use $\LaTeX$ - you can find a tutorial by searching for "latex tutorial" in Google. Here's one, for example. If you want to learn the notation itself, the best way is learning by doing. You should read a mathematics text that's appropriate for your level, and make sure you understand all the notation used there. As you read more complex texts, you will become more and more familiar with the notation. –  Chris Taylor Oct 14 '11 at 11:21
    
So if I download LaTeX and paste your notation then it displays it in a more readable form? –  Dbr Oct 14 '11 at 11:24

For your second question, you know $\Pr(A)$, $\Pr(B)$, and $\Pr(A \text{ and } B)$, so you can work out $\Pr(A \text{ and not } B)$ and $\Pr(B \text{ and not } A)$ by taking the differences. Then add these two together.

Alternatively take $\Pr(A \text{ or } B) - \Pr(A \text{ and } B)$.

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For the additional problem: probability of exactly one equals probability of one or the other but not both, equals probability of union minus probability of intersection, equals $$P(A)+P(B)-2P(A\cap B)$$

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probability of only one event occuring is as follows: if A and B are 2 events then probability of only A occuring can be given as P(A and B complement)= P(A) - P(A AND B )

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