The duration of Alzheimer's disease from the onset of symptoms until death ranges from 3 to 20 years with a mean of 8 and a standard deviation of 4 years (think of these values as population parameters). A study of 81 Alzheimer's patients is collected. What is the probability that the average duration is between 7 and 9 years? (Hint: think of this problem in terms of a sampling distribution with n =81).