Chapter 7 - Confidence Intervals and Sample - 7-1 Confidence Intervals for the Mean When σ Is Known - Exercises 7-1 - Page 379: 10

$16.26\lt\mu\lt20.16$

Given $\bar X=18.21,\sigma=5.92,n=50$, with a 98% confidence, the critical z-value is given by $z_{\alpha/2}=2.326$ and the error is $E=2.326\times\frac{5.92}{\sqrt {50}}=1.95$ Thus the interval for the mean bill for all cars from the drive-thru with 98% confidence is $\bar X-E\lt\mu\lt\bar X+E$ which gives $16.26\lt\mu\lt20.16$