you can play around with these things at
http://davidmlane.com/hyperstat/z_table.html
A bank wishing to determine the average amount of time a customer must wait to be served took a random sample of 100 customers and found that the mean waiting time was 7.2 minutes. Assuming that the population standard deviation is known to be 15 minutes, find the 90% confidence interval estimate of the mean waiting time for all of the bank's customers
2 answers
Fhg