A manager evaluates effectiveness of a major hardware upgrade by running a certain process 40 times before the upgrade and 40 times after it. Based on these data, the average running time is 8.6 minutes before the upgrade, 7.3 minutes after it. Historically, the standard deviation has been 1.8 minutes, and presumably it has not changed. Construct a 90% confidence interval showing how much the mean running time reduced due to the hardware upgrade.