Find a 95% confidence interval for Niyaki’s mean net warranty cost per unit sold (net of the $25 paid for the plan for those who purchase it). You can assume that this mean is for the first failure only; subsequent failures of the same units are ignored here.

Problems 73-76 are related to the data in the files P9_73a.XLS and P9_73b.XLS. The Niyaki Company sells VCRs through a number of retail stores. On one popular model, there is a standard warranty that covers parts for the first 6 months and labor for the first year. Customers are always asked whether they wish to purchase an extended service plan for $25 that extends the original warranty 2 more years—that is, to 30 months on parts and 36 months on service. To get a better understanding of warranty costs, the company has gathered data on 70 VCRs purchased. The variables in the P9_73a.XLS file are:

• @ Extended Plan: whether customer purchased the extended service plan 
• @ Failure Time: time (months) until the first failure of the unit 
• @ Parts Cost: cost of parts to repair the unit 
• @ Labor Cost: cost of labor to repair the unit 

The latter two costs are tracked only for repairs covered by warranty. [Otherwise, the customer bears the cost(s).] The variables in the P9_73b.XLS file are similar, but they also include information of subsequent failures of the units (that occur during the warranty period).