Benson Company bought a machine costing $120,000 on 1 April 2015. Management estimated the useful life to be 10 years and the residual value to be $20,000. On 1 January 2018, management revised the useful life and residual value of the machine. The remaining useful life was revised to 5 years while the residual value was revised to $10,000. The company's financial year end is 31 December. Compute the depreciation expense, accumulated depreciation and net book value of the machine for 2015, 2016, 2017 and 2018. (13 marks)