Math:Probability

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The probability of having homework over the weekend in math is 0.9,history is 0.6, and Spanish is 0.75.
1. What is the probability of having exactly one class that gives homework?
Is this a nCx problem?
I got an illogical answer: 10.54=1054% THIS IS NOT RIGHT!!!!!!!
2. What is the probability of having homework in at least 1 class?
My answer : 0.492

  • Math:Probability -

    1.
    could be 1 in math or 1 in history or 1 in Spanish
    that is (math)(no history)(no Spanish) + ....

    prob = .9(.4)(.25) + .6(.1)(.25) + .25(.1)(.4)
    = .115

    Exclude the case of no homework in any
    which is (.1)(.4)(.25) = .01

    prob(homework in at least 1 class) 1 - .01 = .99

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