According to a poll of 48 randomly chosen "iPhone" users, the average selling price is $390, with a sample standard deviation of $190.

a. Calculate the sample mean's standard error. (Round to the nearest whole number the final answer.)

Standard deviation

b. Calculate the mean's 80 percent confidence interval. (Round the final answers to the nearest two decimal places.)

The trust interval is $$$$$$$$$$$$$$$ as well as $

c. How much of a survey do you need to predict the population mean to within $15 with an 80% confidence level? (Round to the nearest whole number the final answer.)

Scale of the sample