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Probability

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Statistics show that about 5% of all males are colorblind. Suppose that 20 males are selected at random.

What is the probability that at least 4 of the 20 people are colorblind?
My answer: I got 1-0.984
The answer key says 0.00257394

  • Probability -

    at least 4 means
    4 or 5 or 6 or ... or 20 are colourblind

    so exclude cases of none, 1 , 2, and 3

    colourblind = .05
    not colourblind = .95

    None:
    C(20,0) (.05)^0 (.95)^20 = .358486
    one:
    C(20,1) (.05) (.95)^19 = .37735..
    two:
    C(20,2) (.05)^2 (.95)^18 = .1886768
    three:
    C(20,3)(.05)^3 (.95)^17 = .059582..

    adding up those 4 and subtracting from 1 I got .0159
    which is the same result as you had
    mmmhhhh?

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