DEF Inc. purchased equipment for $80,000 on January 1st, 2023. The equipment has an estimated useful life of 5 years and a salvage value of $5,000. Calculate: a) Annual straight-line depreciation expense. b) Accumulated depreciation at the end of the third year. c) Book value of the equipment at the end of the fourth year. d) Gain or loss if the equipment is sold for $50,000 at the end of the fourth year.