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*
Property and Equipment ($ millions)	(Years)	2015	2014
*Weighted averages
Land		$114	$120
Buildings and building equipment	23	3,460	3,037
Machinery and equipment	11	5,499	5,089
Dies, patterns, tools, etc	8	1,567	1,552
All other	5	875	889
Construction in progress		345	530
Total at cost		11,860	11,217
Less accumulated depreciation		5,846	5,694
Total		$6,014	$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 estimated useful life of Deere’s depreciable assets at year-end 2015.

b. Estimate the percent used up of Deere’s depreciable assets at year-end 2015.