The manager of an EZ lube service station has reduced an oil-change job to a science and wants to keep it that way. The manager constantly monitors the performance of his staff. This week, 15 oil-change jobs were sampled with a sample mean of 9.8 minutes per job. Experience has shown that the times follow a normal distribution, and the standard deviation of the population is known to be 1.2 minutes. Based on this week's sample, construct a 90% confidence interval for the population mean (average time for an oil-change job).