A random sample of 20 purchases showed the amounts in the table (in $). The mean is $50.43 and the standard deviation is $19.75. 60.67 26.29 57.32 a) Construct a 80% confidence interval for the mean purchases of all customers, assuming that the assumptions and conditions for the confidence interval 20.76 28.70 37.53 have been met. 63.51 85.02 38.01 b) How large is the margin of error? 44.31 73.79 59.18 c) How would the confidence interval change if you had assumed that the population standard deviation was known to be $20? 44.47 55.54 59.27 44.51 36.18 19.72 65.64 88.20 a) What is the confidence interval? (Round to two decimal places as needed.)