Please answer parts a and b. Please provide explanation if possible. Thanks a lot!

Estimating Useful Life and Percent Used Up The property and equipment footnote from the Deere & Company Equipment and Operations segment follows. Property and Depreciation A summary of property and equipment at October 31 follows. Useful Lives* (Years) 2015 2014 Property and Equipment ($ millions) Land $114 $120 23 2,572 3,037 11 4,611 5,089 Buildings and building equipment Machinery and equipment Dies, patterns, tools, etc All other 8 1,552 1,567 875 5 889 345 530 Support Construction in progress Total at cost Less accumulated depreciation Total *Weighted averages 10,084 11,217 5,846 5,694 $4,238 $5,523 Total property and equipment additions in 2015, 2014 and 2013 were $666 million, $1,016 million and $1,158 million, and depreciation was $692 million, $696 million and $637 million, respectively. a. Compute the average useful life of Deere's depreciable assets at year-end 2015. Round answer to one decimal place. 13.9 x years b. Estimate the percent used up of Deere's depreciable assets at year-end 2015. Round answer to one decimal place (ex: 0.2345 = 23.5%) 0 X %