Oxtongue Limited paid $135,000 to purchase equipment at the beginning of 2020. Oxtongue Limited estimated the useful life of the equipment to be 4 years or 200,000 units. The equipment will be considered fully amortized when the balance in the Accumulated Depreciation account reaches $120,000. The equipment produced 52,000 units in 2023. 

Required 1. Determine the estimated residual value of the equipment. 

2. What is the depreciable cost of the equipment? 

3. Calculate the depreciation expense for 2023 under each of the following methods: 

a. straight-line 

b. units-of-production 

c. double-diminishing-balance