Primary Co. purchased a piece of equipment on January 1,2020 for $18,000. They estimated a total useful life of 30,000 operating hours and a residual value of $3,000. Use the units of activity method for depreciation. A. Primary operated the equipment for 3,000 hours in 2020 . Calculate depreciation expense for the year. B. Primary operated the equipment for 4,000 hours in 2021 . Calculate depreciation expense for the year