# Homework Help: Find the probabilities

1. Sep 5, 2013

Finding probabilities through general addition rule

1. The problem statement, all variables and given/known data
http://postimg.org/image/q9ztktpot/ [Broken]

2. Relevant equations
P(A or B)=P(A)+P(B)-P(A and B)
P(A and B)=P(A)P(B)

3. The attempt at a solution
The question is for part b). It said it was only focused on the mens so i ignored all the other numbers.
I used the general addition rule where P(A or B)=P(A)+P(B)-P(A and B). Since these events are independent, i used this equation... P(A and B)=P(A)P(B). I found P(A) from 400/2000= 0.2
I found P(B) from 500/2000=0.25. Then i found P(A and B)=P(A)P(B)=0.05

Now i plugged it into the general addition rule... P(A or B)=0.2+0.25-0.05=0.4 but it was wrong.
What did i do wrong?

Last edited by a moderator: May 6, 2017
2. Sep 5, 2013

### MrAnchovy

The events are not independent, they are exclusive; if a man likes the commercial the probability that he is neutral is zero.

But you do not need probabilty theory to answer (b) anyway - it is a simple percentage question.

3. Sep 5, 2013

Would it just be (400+500)/2000 =0.45?

Last edited: Sep 5, 2013
4. Sep 5, 2013

### MrAnchovy

Yes :)

5. Sep 5, 2013

is it because the two varibles cant overlap because their exclusive so we dont have to account for the everlap so we dont need to use the inclusion-exclusion rule?

6. Sep 5, 2013

### Ray Vickson

Your question is impossible to answer, since you do not tell us what you mean by A and B, etc.

7. Sep 5, 2013