4. [Calculation of depreciation] Nobita Popsicle Company purchased an ice maker on July 1, x5, at a cost of $315,000, an estimated service life of 5 years, and a residual value of $15,000. The total working time of the ice maker is 30,000 machine hours: x 6 years of actual use of 7.200 machine hours. Try to calculate the depreciation amount for the year x6 according to the following methods, and make relevant adjustment (1) Total years method. (2) Double declining balance method.