# Find the probabilities

Finding probabilities through general addition rule

## Homework Statement

http://postimg.org/image/q9ztktpot/ [Broken]

## Homework Equations

P(A or B)=P(A)+P(B)-P(A and B)
P(A and B)=P(A)P(B)

## 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?

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pbuk
Gold Member
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.

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

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pbuk
Gold Member
Yes :)

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?

Ray Vickson