7.21 A random sample of 16 tires was tested to estimate the average life of this type of tire under normal driving conditions. The sample mean and sample standard deviation were found to be 47,500 miles and 4,200 miles, respectively. a. Calculate the margin of error for a 95% confi dence interval estimate of the mean lifetime of this type of tire if driven under normal driving conditions. b. Find the UCL and the LCL of a 90% confidence interval estimate of the mean lifetime of this type of tire if driven under normal driving conditions.