# How many days does it rain in May?

1. Aug 25, 2014

### Scienceboy1776

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. Aug 25, 2014

### Ray Vickson

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"?

3. Aug 29, 2014

### 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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