Chapter 5 - Continuous Random Variables - Exercises 5.20 - 5.52 - Learning the Mechanics - Page 242: 5.33a

$x=-1.28(8)+30=19.76$

Given the symmetry of the standard normal curve: $p(z\lt z_0)=p(z\gt -z_0)$ According to 5.32c If $p(z\gt -z_0)=.10$ then $-z_0=1.28$ We can conclude that If $p(z\lt z_0)=.10=10$% then $z_0=-1.28$ $z_0=\frac{x-\mu}{\sigma}$ $-1.28=\frac{x-30}{8}$ $x=-1.28(8)+30=19.76$