8.18 The file FastFood contains the amount that a sample of 15 customers spent for lunch ($) at a fast-food restaurant: 7.42 6.29 5.83 6.50 8.34 9.51 7.10 6.80 5.90 4.89 6.50 5.52 7.90 8.30 9.60 a. Construct a 95% confidence interval estimate for the population mean amount spent for lunch ($) at a fast-food restaurant, assuming a normal distribution. b. Interpret the interval constructed in (a).