Say there is a $20$ percent chance of rain on Saturday, $30$ percent chance of rain on Sunday. If the two events are independent of each other, then the chance of it raining on at least one of the days is $$.2 + .3 - (.2)(.3) = .44.$$ However, if I'm told that raining on Saturday makes it more likely to rain on Sunday, will the probability go ... More @Wikipedia
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