A confidence interval for the true mean of the annual medical expenses of a middle-class American family is given as ($ 739, $ 781). If this interval is based on interviews with 80 families and a standard deviation of $ 130 is assumed. Suppose all annual medical expenses of middle-class American families follow an approximately normal distribution.
(a) What is the sample mean of annual medical expenses?
(b) What is the confidence level of the interval estimate (as a decimal)?