Given these probabilities: pr(rain) = .2
pr(snow)=.1
pr(cloudy)=.5
and pr(sun)=.2
pr(crash|rain or snow) = .2
pr(crash|cloudy or sun) = .05
what is the probability of a car crash?
If the events are independent, the probability of both/all events occurring is determined by multiplying the probabilities of the individual events.
Either-or probabilities are found by adding the individual probabilities.
Since pr(crash|rain or snow) = .2 and pr(crash|cloudy or sun) = .05, this is either/or.