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 21.7 miles per gallon (mpg) with a sample standard deviation of 0.9 mpg. (Uset Distribution Table & z Distribution Table.) Required: a. Compute the 99% confidence interval for his mpg. (Round your answers to 3 decimal places.) b. To be 99% confident, how many times should he fill his gas tank to obtain a margin of error below 0.15 mpg? (Round your answer to the next whole number.)