1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    Mark44

    Staff: Mentor

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

    "Thirty days hath September, April, June, and November..."
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: How many days does it rain in May?
  1. How many combinations? (Replies: 14)

Loading...