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Homework Help: Find the probabilities

  1. Sep 5, 2013 #1
    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. jcsd
  3. Sep 5, 2013 #2
    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.
  4. Sep 5, 2013 #3
    Would it just be (400+500)/2000 =0.45?
    Last edited: Sep 5, 2013
  5. Sep 5, 2013 #4
    Yes :)
  6. Sep 5, 2013 #5
    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?
  7. Sep 5, 2013 #6

    Ray Vickson

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    Science Advisor
    Homework Helper

    Your question is impossible to answer, since you do not tell us what you mean by A and B, etc.
  8. Sep 5, 2013 #7
    Let A be the males that like the commercial and B, the male that are neutral towards the commercial
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