Playbill magazine reported that the mean annual household income of its readers is $120,000 (i.e., this figure is a sample mean). Assume that this mean is based upon a sample of size n = 25. Find a 90% confidence interval estimate for the population mean, under the following two conditions (the two parts are completely independent of each other):

a) The population standard deviation of household incomes is 20,000.

b) The population is not known, but the sample standard deviation s of incomes is 22,000.