7.29 Build Rental is a plant hire company in the United Kingdom that provides machinery, equipment, and tools for a limited period to construction contractors.

It is interested in the amount of time its equipment are out of operation for repair work. State all assumptions and find a 90% confidence interval for the mean number of days in a year that all Build Rental’s equipment are out of operation if a random sample of nine pieces of equipment showed the following number of days that each had been inoperative:

16 15 11 26 18 23 8 8 24 14