An owner of a popular café wants to know how long customers stay at their tables. Over the course of a month, she asks her employees to keep track of a random sample of customers who stay and use the tables and record how long they sit at a table after making a purchase. The sample of 41 customers stay at the tables using the free wifi for 100 minutes on average with a standard deviation of 30 minutes. Estimate a 95% confidence interval for the population mean.