Dylan Jones kept careful records of the fuel efficiency of his new car. After the first eleven times he filled up the tank, he found the mean was 23.4 miles per gallon (mpg) with a sample standard deviation of 1.4 mpg.
(a) Compute the 95 percent confidence interval for his mpg. (Round your answers to 3 decimal places.)
(b) How many times should he fill his gas tank to obtain a margin of error below 0.15 mpg?