Answer
$26.6\lt\mu\lt29.6$
Work Step by Step
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
$26.6\lt\mu\lt29.6$