a study of two kinds of machine failures shows that 58 failures of the first kind took on the average 79.7 minutes to repair with a sample standard deviation of 18.4 minutes, whereas 71 failures of the second kind took on average 87.3 minutes to repair with a sample standard deviation of 19.5 minutes. find a 99% confidence interval for the difference between the true average amounts of time it takes to repair failures of the two kinds of machines.