A random sample of 105 light bulbs had a mean life of 441 hours with a standard deviation of 40 hours. Construct a 90% confidence interval for the mean life, mu, of all light bulbs of this type.

A z-score of ~1.645 will give a two-tailed confidence level of ~0.9 = 90%. z = (x-μ) / σ

z∗σ=|x-μ|

Where z is the z-score, sigma is the standard deviation, / mu is the mean, and x is the picked score. Solve for both values of x.