4295% CI for the mean

A sample of 25 has mean 50; $\displaystyle \sigma = 10$ is known. Find the 95% confidence interval for the mean ($\displaystyle z = 1.96$).

With σ known, use x̄ ± z·(σ/√n). The standard error is 10/√25 = 2, so the margin is 1.96×2 = 3.92. The 95% CI is (46.08, 53.92).