Solution;
[tex]\begin{gathered} n=81 \\ x\text{ \lparen mean\rparen=13} \end{gathered}[/tex]To determine Proportion;
[tex]p=\frac{x}{n}[/tex][tex]\begin{gathered} p=\frac{13}{81} \\ p=0.1604938 \end{gathered}[/tex]To determine the standard error;
[tex]\begin{gathered} SE=\frac{standard\text{ deviation}}{\sqrt{n}}=\frac{\sqrt{npq}}{\sqrt{n}} \\ where\text{ q=1-p} \end{gathered}[/tex][tex]SE=\frac{3.30357}{9}=0.36706[/tex]To determine 90% confidence interval;
[tex]\begin{gathered} C.I=x\pm z(standard\text{ error}) \\ where\text{ z is the z score at 90}\%\text{ confidence interval.} \end{gathered}[/tex][tex]\begin{gathered} C.I=13\pm1.645(0.36706) \\ C.I=13\pm0.6038137 \\ 13-0.6038136<\text{ 13+0.6038136} \\ 12.3962<13.6038\text{ to 4 d.p} \end{gathered}[/tex]Lower bound is 12.3962
Upper bound is 13.6038