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March 29, 2017

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12. The proportion of students who own a cell phone on college campuses across the country has increased tremendously over the past few years. It is estimated that approximately 94% of students now own a cell phone. Twenty five students are to be selected at random from a large university. Assume that the proportion of students who own a cell phone at this university is the same as nationwide. Let X = the number of students in the sample of 15 who own a cell phone.

QUESTION: What is the probability that if you choose 40 people exactly 38 will have cell phones?

  • Statistics - ,

    "Let X = the number of students in the sample of 15 who own a cell phone. "
    does not seem to be relevant to the question.

    Situation:
    1. Each outcome is a Bernoulli trial, i.e. success (owns a cell phone) or failure (does not own one)
    2. The probability of success is constant throughout the trials (p=0.94, q=1-p=0.06)
    3. Every trial has equal probability as the others.
    4. Every trial is independent of the others
    5. There is a fixed number of trials (n=40)

    Under these conditions, the probability can be modelled using the binomial distribution.
    Let i=number of successes (n-i=#of failures0, then
    P(i)=C(n, i)piqn-i
    and C(n, i) is the combination function, where
    C(n,i)=n!/(i!(n-i)!)
    Example:
    for n=40, i=38, p=0.99, q=0.03
    I get P(38)=0.2206

  • Statistics - correction - ,

    Sorry, for the example, p=0.97, q=0.03.

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