Use the next formula to find the margin or error:
[tex]MOE=z*\frac{\sigma}{\sqrt{n}}[/tex]z is the critical value (For a 95% confidence level the z-score is 1.95998)
σ is the standard deviation
n is the sample size
[tex]\begin{gathered} MOE=1.95998*\frac{20.4}{\sqrt{36}} \\ \\ MOE\approx6.6639 \end{gathered}[/tex]