Problem N5631 probability theory confidence interval

5631. The time spent on the manufacture of one part is a random variable distributed according to the normal law. In a random survey of 100 workers, it was revealed that an average of 6 minutes was spent on the manufacture of one part. Find with a reliability of 0.99 a confidence interval for estimating the true time spent a for the manufacture of one part, if its standard deviation σ=0.5 min.