## An investigator compares the durability of two different compounds used in the manufacture of a certain automobile brake lining

An investigator compares the durability of two different compounds used in the manufacture of a certain automobile brake lining. A sample of 199 brakes using Compound 1 yields an average brake life of 47,673 miles. A sample of 157 brakes using Compound 2 yields an average brake life of 48,245 miles. Assume the standard deviation of brake life is known to be 3815 miles for brakes made with Compound 1 and 1538 miles for brakes made with Compound 2. Determine the 80%

confidence interval for the true difference between average lifetimes for brakes using Compound 1 and brakes using Compound 2.

a. Find the critical value that should be used in constructing the confidence interval.

b. Construct the 80% confidence interval. Round your answers to the nearest whole number.