A corporation has a fleet of 480 company cars-100 compact, 180 midsize, and 200 full size. To estimate the overall mean annual repair costs for these cars, a preliminary random sample of 10 cars of each type is selected.
The sample standard deviations for repair costs are $105 for compact cars, $162 for midsize cars, and $183 for full-size cars. A 95% confidence interval for the overall population mean annual repair cost per car that extends $20 on each side of the sample point estimate is required. Estimate the smallest total number of additional sample observations that must be taken.