Subjects statistics

Probability Sample Mean 683C3C

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1. **Stating the problem:** We want to find the probability that the sample mean exceeds 130 days. 2. **Formula and explanation:** When dealing with sample means, the sampling distribution of the sample mean \(\bar{X}\) is approximately normal if the sample size is large, with mean \(\mu\) and standard error \(\sigma_{\bar{X}} = \frac{\sigma}{\sqrt{n}}\). 3. **Calculate the standard error:** Suppose the population mean is \(\mu\) and population standard deviation is \(\sigma\), and sample size is \(n\). 4. **Find the z-score:** $$ z = \frac{\bar{X} - \mu}{\sigma / \sqrt{n}} $$ where \(\bar{X} = 130\). 5. **Calculate the probability:** The probability that the sample mean exceeds 130 days is $$ P(\bar{X} > 130) = P\left(Z > z\right) = 1 - P(Z \leq z) $$ where \(Z\) is a standard normal variable. 6. **Interpretation:** Use standard normal tables or software to find \(P(Z \leq z)\) and subtract from 1 to get the final probability. *Note:* Since the problem does not provide \(\mu\), \(\sigma\), or \(n\), the exact numerical answer cannot be computed here. **Final answer:** The probability that the sample mean exceeds 130 days is \(P(\bar{X} > 130) = 1 - \Phi\left(\frac{130 - \mu}{\sigma / \sqrt{n}}\right)\), where \(\Phi\) is the standard normal cumulative distribution function.