## Elementary Statistics (12th Edition)

Given mean:23.95. Given $\sigma$:2.55. $\alpha=1-0.98=0.01.$ $\sigma$ is known, hence we use the z-distribution using the table. $z_{\alpha/2}=z_{0.005}=2.58.$ Margin of error:$z_{\alpha/2}\cdot\frac{\sigma}{\sqrt {n}}=2.58\cdot\frac{2.55}{\sqrt{40}}\approx1.04.$ Hence the confidence interval:$\mu$ is between 23.95-1.04=22.91 and 23.95+1.04=24.99.