A manager is interested in the amount of time employees spend each day playing video games at work. The manager takes a random sample of 18 employees who showed a mean time of 55 minutes per day with a standard deviation of 5 minutes. Assuming that these times follow a normal distribution, compute a 90% confidence interval for the average amount of time employees spend playing video games each day at work.

How come this question requires T distribution when I have all the pieces to calculate it normally?