On a particular day in July, there is a 20% probability of rain in Vancouver, a 65% probability of rain in Calgary, and a 75% probability of rain in Saskatoon. What is the probability that it will rain in all 3 cities on that day?

The probability that it will  rain in all 3 cities on that day

P(V∪ C∪S) = 0.93 = 93%

Step-by-step explanation:

Let 'V' be the event of  rain in Vancouver

Given that the probability of rain in Vancouver

             P( V) = 20% = 0.20

             P( V⁻) = 1- P( V) = 1 - 0.20 =0.80

Let 'C' be the event of  rain in Calgary

Given that the probability of rain in Calgary

             P( C) = 65% = 0.65

            P( C⁻) = 1- P( C) = 1 - 0.65 =0.35

Let 'S' be the event of  rain in Saskatoon

Given that the probability of rain in Saskatoon

             P( S) = 75% = 0.75

             P( S⁻) = 1- P( S) = 1 - 0.75 =0.25

First we will prove that the probability that it will not rain in all 3 cities on that day

P( V⁻ ∩  C⁻ ∩  S⁻  ) =  P( V⁻) P( C⁻) P(S⁻)

                              =  0.80 × 0.35 × 0.25

                             = 0.07

The probability that it will  rain in all 3 cities on that day

P(V∪ C∪S) = 1-(P( V⁻ ∩  C⁻ ∩  S⁻  )

                   = 1 - 0.07

                  = 0.93

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