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    Undergrad Probability airplane returns in 30 or 40 minutes given not back in 20

    hi all, I have a question that i really want to know the answer. the probability that an airplane will return in 10min is 30%, in 20min is 30%, in 30min is 0%, in 40min is 40% according to the past history records. given that the airplane has not return in 20min, what is the probability...