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Simple Probability Problem

  1. Mar 3, 2009 #1
    1. The problem statement, all variables and given/known data

    A die is biased such that the probability of getting a six is 1/4. The die is rolled 2000 times. Let X be the number of sixes obtained. Find,

    a) the mean of X
    b) the standard deviation of X, leaving your answer as a surd.

    2. Relevant equations

    3. The attempt at a solution

    a) The mean of X is simple, given P(6)=1/4, and the dice is rolled 2000 times, the mean of X is 2000(1/4)=500

    b) This is where I don't know what to do. Any guidance?
  2. jcsd
  3. Mar 3, 2009 #2


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