Solution
= Solution
Let $z_p$ denote the $p$-quantile of the standard normal distribution. Since the variance is known,
$$
\frac{\sqrt n(\overline X-\mu)}{\sigma}\sim N(0,1),
$$
so a $100(1-\alpha)\%$ <confidence interval> is
$$
\boxed{\overline X\pm z_{1-\alpha/2}\frac\sigma{\sqrt n}}.
$$