Dylan Jones kept careful records of the fuel efficiency of his new car. After the first nine times he filled up the tank, he found the mean was 23.4 miles per gallon (mpg) with a sample standard deviation of 0.9 mpg.
a. Compute the 95% confidence interval for his mpg.
b. How many times should he fill his gas tank to obtain a margin of error below 0.1 mpg?