help please.

The duration of Alzheimer's disease from the onset of symptoms to death varies from 3 to 20 years; the mean is 8 years with a standard deviation of 4 years. The administrator of a large medical center randomly selects the medical records of 50 deceased Alzheimer's patients from the medical center's database and notes the average duration. Find the approximate probabilities for these events: a. The average duration is less than 7 years. b. b. La duracion promedio excede de 9 aos