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Homework Help: Probability of an event occurring at least once over n trials

  1. Mar 18, 2013 #1
    1. The problem statement, all variables and given/known data
    There is a 1 in 4 chance that on any given sexual encounter, Joe will contract an STD. If Joe has sex 4 times, what are the chances of him having an STD?

    2. Relevant equations

    Something with combinations maybe?

    3. The attempt at a solution

    I thought you would just add up the probability of the event happening for each trial, but that would give me 100% which isn't right.
  2. jcsd
  3. Mar 18, 2013 #2


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    Homework Helper

    Something simpler. What is the complementary event to "getting STD from at least one encounter"?
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