A random sample of 18 health maintenance organizations (HMOs) was selected. For each HMO, the co-payment (in dollars) for a doctor's office visit was recorded. The results are as follows. 12, 7, 11, 12, 8, 10, 5, 8, 9, 11, 8, 10, 12, 5, 6, 6, 9, 9 Send data to calculator Send data to Excel Under the assumption that co-payment amounts are normally distributed, find a 90% confidence interval for the mean co-payment amount in dollars. Then find the lower limit and upper limit of the 90% confidence interval. Carry your intermediate computations to at least three decimal places. Round your answers to one decimal place. (If necessary, consult a list of formulas.) Lower limit: I X ? Upper limit