申論題

3. (總分 10 分) The duration of a disease from the onset of symptoms until death ranges from 3 to 20 years; the average is 8 years with a standard deviation of 4 years. A researcher randomly selects the medical records of 30 patients who died as a result of this disease from the medical center's database, and records the average duration.

(b). (5 分) Find the approximate probabilities for an event where average duration exceeds 7 years.