The property and equipment footnote for Deere & Company's Equipment Operations segment follows $($ $ in millions$).$

Depreciation expense for years $2019$ and $2018$ was $$779$ million and $$754$ million, respectively. Assume straight$-$line depreciation and zero

salvage value for all depreciable assets.

a$.$ Calculate the average useful life of the company's depreciable assets as of year$-$end $2019.$