# [SOLVED] Calculate the following probabilities

• Oct 12th 2008, 05:54 AM
the1u2001
[SOLVED] Calculate the following probabilities
Let A and B be events with P(A) = 2/5, P(B) = 1/2, P(AnB) = 3/20. Calculate the
following probabilities.
i) P(Ac) (means probabilty of Not A)
ii) P(A U B)
iii) P(Ac n B)
iv) P(B\A)

I know:
1) P(Ac)= 1-2/5= 3/5
2)P(A U B)= 2/5+1/2-3/20 = 3/4

But i am stuck on
iii) P(Ac n B)
iv) P(B\A)

• Oct 12th 2008, 06:23 AM
Plato
How are you using P(A\B)? Do you mean A not B or A given B?
In either case you are mistaken about the value $\displaystyle P(A \cap B)$ unless that is what P(A\B).
• Oct 12th 2008, 06:54 AM
the1u2001
Quote:

Originally Posted by Plato
How are you using P(A\B)? Do you mean A not B or A given B?
In either case you are mistaken about the value $\displaystyle P(A \cap B)$ unless that is what P(A\B).

Yeah sorry about that i meant http://www.mathhelpforum.com/math-he...a2e41cef-1.gif= 3/20
• Oct 12th 2008, 07:21 AM
Plato
$\displaystyle \begin{gathered} P(B) = P(B \cap A) + {\color{blue}P(B \cap A^c )} \hfill \\ {\color{blue}P(B \cap A^c )} = P(B) - P(B \cap A) \hfill \\ \end{gathered}$