On January 1, 2022, a company purchased a machine for $100,000. The company depreciates the machine on a straight-line basis over its useful life of 5 years, with no residual value. The company uses the calendar year as its fiscal year. On December 31, 2024, the company decides to revise its estimate of the useful life of the machine to 4 years from the original estimate of 5 years.

a) Calculate the annual depreciation expense of the machine under the original estimate.
b) Calculate the book value of the machine on December 31, 2024, under the original estimate.
c) Calculate the revised annual depreciation expense of the machine.
d) Calculate the revised book value of the machine on December 31, 2024.