Given
[tex]\begin{gathered} n=21 \\ \bar{x}=39 \\ s=8 \\ C.I=90\text{\%=1.645} \end{gathered}[/tex]Explanation
The margin of error formula is given as;
[tex]M.E=z_{\frac{a}{2}}\times\frac{s}{\sqrt{n}}[/tex]Therefore, we will have;
[tex]\begin{gathered} M.E=1.645\times\frac{8}{\sqrt{21}} \\ M.E=2.87 \\ \end{gathered}[/tex]Answer: 2.87
