The city fuel economy of a 2009 Toyota 4Runner 2WD 6 cylinder 4 L automatic 5-speed using regular gas is a normally distributed random variable with a range 16 MPG to 21 MPG.
(a) Estimate the standard deviation using Method 3 (the Empirical Rule for a normal distribution).
(b) What sample size is needed to estimate the mean with 90 percent confidence and an error of ±0.25 MPG?