## Essentials of Statistics (5th Edition)

Although the dot plot is not roughly bell-shaped, the sample size is more than 30 (it is 40), hence the sample data is roughly normally distributed, hence $\alpha=1-0.99=0.01.$ $\sigma$ is not known, hence we us the t-distribution with $df=sample \ size-1=40-1=39$ in the table. $t_{\alpha/2}=t_{0.005}=2.708.$