Find the probabilities

  • Thread starter maiad
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  • #1
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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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Answers and Replies

  • #2
pbuk
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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
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Would it just be (400+500)/2000 =0.45?
 
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  • #4
pbuk
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Yes :)
 
  • #5
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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
Ray Vickson
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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?

Your question is impossible to answer, since you do not tell us what you mean by A and B, etc.
 
  • #7
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Let A be the males that like the commercial and B, the male that are neutral towards the commercial
 

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