Chapter 7 - Confidence Intervals and Sample - 7-1 Confidence Intervals for the Mean When σ Is Known - Exercises 7-1 - Page 380: 19

$59.5\lt\mu\lt62.9$

Given $\bar X=61.2, \sigma=7.9, n=84$, at a 95% confidence, the critical z-value is $z_{\alpha/2}=1.96$ and the error of estimation can be found as $E=1.96\times\frac{7.9}{\sqrt {84}}=1.69$ Thus, the interval of the true mean can be found as $\bar X−E\lt\mu\lt\bar X+E$ which gives $59.5\lt\mu\lt62.9$