A steel company is planning to produce cold reduced sheet steel consisting of .15 percent copper at an annealing temperature of 1,150 (degrees F), and is interested in estimating the average (Rockwell 30-T) hardness of a sheet. To determine this, they have collected the data shown in Table 9.6 on 10 different specimens of sheet steel having different copper contents and annealing temperatures. Estimate the average hardness and determine an interval in which it will lie with 95 percent confidence.