Swindall Industries uses straight-line depreciation on all of its depreciable assets. The company records annual depreciation expense at the end of each calendar year. On January 11, 2017, the company purchased a machine costing $90,000. The machine’s useful life was estimated to be 12 years with an estimated residual value of $18,000. Depreciation for partial years is recorded to the nearest full month. In 2021, after almost five years of experience with the machine, management decided to revise its estimated life from 12 years to 20 years. No change was made in the estimated residual value. The revised estimate of the useful life was decided prior to recording annual depreciation expense for the year ended December 31, 2021. 

a. Prepare journal entries in chronological order for the given events, beginning with the purchase of the machinery on January 11, 2017. Show separately the recording of depreciation expense in 2017 through 2021. 

b. What factors may have caused the company to revise its estimate of the machine’s useful life?