# Thread: Basic Mutually exclusive question :S

1. ## Basic Mutually exclusive question :S

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.

2. Originally Posted by steph_r
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

3. ## mutually exclusive

Originally Posted by CaptainBlack
When $A$ and $B$ are mutually exclusive $A' \cap B = B$

CB
How do you know that though??

4. Originally Posted by steph_r
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