The duration of Alzheimer's 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. The administrator of a large medical centre randomly selects the medical records of 30 deceased Alzheimer's patients from the medical centre's database and records the aver- age duration. Find the approximate probabilities for these events: 1. The average duration is less than 7 years. 2. The average duration exceeds 7 years. 3. The average duration lies within 1 year of the population mean 8.