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Homework Help: How many days does it rain in May?

  1. Aug 25, 2014 #1
    1. The problem statement, all variables and given/known data

    A survey concludes that it in average rains 15 of 30 days in may.

    Thusly P(R|M) = 1/2

    Where M is May and R is rain.

    The Weatherservice forcasts fits 2/3 of the time. Thus it does not rain 2/3 of the time.

    I bring umbrella with me every day the promise and 1/3 of the days where do not perdict rain.

    1) What is the prob that that Weather Service promises dry weather in a day in May?

    2) What is the prob that it rains on a day where I did not bring my umbrella?

    2. Relevant equations

    P(A|B) = P(A n B)/P(B)

    3. The attempt at a solution

    By using the formula in condition prob.

    I get 1)

    P(R|M) = 1/2 P(R) = P(R) = 1 - P(does not rain) = 1 - 1/3 = 2/3

    Thus 1/2 = P(R n M) / P(R) -> 1/2 = (R n M)/ (2/3) = 1/3

    I am not sure about 2 ?

    2) P = 1/3 * 2/3 = 2/9 ?

    Hope there is someone who would take a look at it?
  2. jcsd
  3. Aug 25, 2014 #2

    Ray Vickson

    User Avatar
    Science Advisor
    Homework Helper

    Are you saying that P(Rain|forcast rain) = P(no rain|forecast no rain) = 2/3, or are you saying P(forecast rain|rain) = P(forecast no rain|no rain) = 2/3? Do you see the difference? Anyway, your conclusion "Thus it does not rain 2/3 of the time" is wrong; do you see why?

    Your sentence "I bring umbrella with me every day the promise and 1/3 of the days where do not perdict rain" is so badly written as to be almost incomprehensible; do you mean that you "bring an umbrella whenever the forecast is for rain and also 1/3 of the time when the prediction is for no rain"?
  4. Aug 29, 2014 #3


    Staff: Mentor

    By the way, May has 31 days in it, not 30.

    "Thirty days hath September, April, June, and November..."
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