Chapter 7 - Confidence Intervals and Sample - 7-1 Confidence Intervals for the Mean When σ Is Known - Exercises 7-1 - Page 379: 9

$26.6\lt\mu\lt29.6$

Given $\bar X=28.1,\sigma=4.7,n=40$, the critical z-value for a confidence level $c=0.95$ is $z_{\alpha/2}=1.96$ and the error is given by $E=1.96\times\frac{4.7}{\sqrt {40}}=1.46$ Thus, the interval for the true mean gas mileage with 95% confidence is $\bar X - E\lt\mu\lt\bar X+E$ which gives $26.6\lt\mu\lt29.6$