Chapter 11 Data Analysis and Statistics - 11.3 Use Normal Distributions - 11.3 Exercises - Skill Practice - Page 760: 9

$16\%$

As we can see, these are half the elements that are at least $1$ standard deviation away from the mean. The normal distribution is symmetric about the mean, thus using the 68-95-99.7 rule and the graph we get: $P(\overline{x}+\sigma\leq x)=13.5\%+2.35\%+0.15\%=16\%$