1. **Problem Statement:** We need to find a 90% confidence interval for the standard deviation of the weights of single serving size packages of Reese's Cups. 2. **Formula and Explanation:** The confidence interval for the standard deviation $\sigma$ when the population is normally distributed is based on the chi-square distribution. The formula for the confidence interval is: $$\left(\sqrt{\frac{(n-1)s^2}{\chi^2_{\alpha/2, n-1}}}, \sqrt{\frac{(n-1)s^2}{\chi^2_{1-\alpha/2, n-1}}}\right)$$ where: - $n$ is the sample size, - $s$ is the sample standard deviation, - $\chi^2_{\alpha/2, n-1}$ and $\chi^2_{1-\alpha/2, n-1}$ are the critical values from the chi-square distribution with $n-1$ degrees of freedom, - $\alpha = 1 - 0.90 = 0.10$ for a 90% confidence level. 3. **Conditions for Using This Formula:** The formula can be used if the sample comes from a normally distributed population. This is important because the chi-square distribution for variance relies on normality. 4. **Steps to Calculate:** - Determine $n$ and calculate $s$ from the sample data. - Find the chi-square critical values $\chi^2_{0.05, n-1}$ and $\chi^2_{0.95, n-1}$. - Plug values into the formula to get the confidence interval for $\sigma$. 5. **Interpretation:** The 90% confidence interval means we are 90% confident that the true population standard deviation of the weights lies within this interval. Since no specific sample data is provided, the exact numerical interval cannot be calculated here.