SmithCo is a supplier to many brand-name makers of mobile phones. SmithCo manufactures the external shells that enclose mobile phones. The strength of a shell is measured by applying increasing pressure to the shell and recording the pressure at which the shell breaks. Shell strengths are approximately normally distributed. A random sample of 10 shells made by SmithCo showed an average strength of 45.0 pounds and a standard deviation of 2.0 pounds. Use StatTools, or conduct calculations by hand, to construct a 90% confidence interval for the mean shell strength. The appropriate multiplier is 1.833.

a. State the confidence interval in the form a ± b.

b. State the confidence interval in the form [c, d].