can someone please help me? i'm stuck.

4. Lone Star Supply Company purchased equipment on January 1, 2017 for $90,000. It is estimated that the equipment will have a $5,000 salvage value. It is also estimated that the equipment will produce 100,000 units over its 5-year useful life. If 16,000 units of product are produced in 2017 and 24,000 units are produced in 2018, what is the book value of the equipment at December 31, 2018? The company uses the units-of-production depreciation method