The average selling price of a smartphone purchased by a random sample of 44 customers was $303. Assume the population standard deviation was $31. a. Construct a 90% confidence interval to estimate the average selling price in the population with this sample. b. What is the margin of error for this interval? a. The 90% confidence interval has a lower limit of $ and an upper limit of $ (Round to the nearest cent as needed.) b. The margin of error is $ (Round to the nearest cent as needed.)