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Calculating mean and variance of probability

  1. Feb 14, 2012 #1
    1. The problem statement, all variables and given/known data

    Given a 30% failure rate, what is the mean number of students who fail, and what is the variance? There are 8 students in the course.

    2. Relevant equations

    I think these are the correct equations:
    Mean = Probability * Number of students
    Variance = Sum from i=1 to 8 of pi(xi-Mean)^2

    3. The attempt at a solution

    So using the Mean formula I got 0.3*8=2.4
    Using the variance formula I got
    (0.3)(1-2.4)2+(0.3)(2-2.4)2+(0.3)(3-2.4)2+(0.3)(4-2.4)2+(0.3)(5-2.4)2+(0.3)(6-2.4)2+(0.3)(7-2.4)2+(0.3)(8-2.4)2
    which is approximately equal to 23.184 (see variance formula here: wolfram alpha, then click approximate form )
    Do I then square root to find the actual variance?

    Thanks for any help!
     
  2. jcsd
  3. Feb 14, 2012 #2

    lanedance

    User Avatar
    Homework Helper

    the pi in your variance equation are not correct, you need to have a think about the probability distribution
     
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