We have that the empirical rule to construct a 68% confidence interval can be expressed with the following general rule:
[tex](\mu-\sigma,\mu+\sigma)[/tex]in this case, we have the following information:
[tex]\begin{gathered} \mu=312 \\ \sigma=30 \end{gathered}[/tex]then, the confidence interval with 68% of confidence is:
[tex](312-30,312+30)=(282,342)[/tex]therefore, the confidence interval with 68% of confidence is (282,342)