Basic Mutually exclusive question :S

steph_r · #1 $Old$ November 3rd, 2009, 11:04 PM

Hello everyone.
I have an exam in a couple of days, and i came across some basics that i have completely forgotten ..

The question is:

Two events, A and B, from a given event space, are such that Pr(A) = 1/5 and Pr(B) = 1/3.

1. Calculate Pr(A' intersection B) when Pr(A intersection B) = 1/8
I've worked this out, and got 5/24

2. Calculate Pr(A' intersection B) when A and B are mutually exclusive.
Im not quite sure how to approach this one

Any help would be appreciated.

CaptainBlack · #2 $Old$ November 4th, 2009, 12:12 AM

Quote:

Originally Posted by steph_r $View Post$

Hello everyone.
I have an exam in a couple of days, and i came across some basics that i have completely forgotten ..

The question is:

Two events, A and B, from a given event space, are such that Pr(A) = 1/5 and Pr(B) = 1/3.

1. Calculate Pr(A' intersection B) when Pr(A intersection B) = 1/8
I've worked this out, and got 5/24

2. Calculate Pr(A' intersection B) when A and B are mutually exclusive.
Im not quite sure how to approach this one

Any help would be appreciated.

When $A$ and $B$ are mutually exclusive $A' \cap B = B$

CB

steph_r · #3 $Old$ November 4th, 2009, 12:19 AM

Quote:

Originally Posted by CaptainBlack $View Post$

When $A$ and $B$ are mutually exclusive $A' \cap B = B$

CB

How do you know that though??

CaptainBlack · #4 $Old$ November 4th, 2009, 12:32 AM

Quote:

Originally Posted by steph_r $View Post$

How do you know that though??

Draw a Venn diagram.

Or suppose that $x \in B$ , then as $A$ and $B$ are mutually exclusive $A\cap B=\emptyset$ so $x \not\in A$ hence $x \in A'$ . Hence every $x\in B$ is in $A'$ so $A'\cap B=B$

CB