statistics

#1
Mileage tests are conducted for a particular model of automobile by testing twenty cars and calculating a sample mean of thirty miles per gallon with a sample standard deviation of four miles per gallon. It is known from prior research that car mileages are normally distributed.

· Determine a 95% confidence interval for the population mean.
Lower Confidence Limit .
Upper Confidence Limit .

· Determine a 95% confidence interval, if the population standard deviation was known to be four miles per gallon.

Lower Confidence Limit .
Upper Confidence Limit .

How large a sample would be required to determine a 98% confidence interval to a precision of plus/minus one mile per gallon?