On January 1, 2022, a company purchased a machine for $500,000. The machine has a useful life of 10 years and an estimated salvage value of $50,000. The company expects the machine to generate annual net cash flows of $100,000 for the next 10 years. The company uses straight-line depreciation for all of its assets. The company's required rate of return is 12%.

a) Calculate the book value of the machine at the end of year 4.
b) Calculate the annual depreciation expense.
c) Calculate the net present value of the cash flows generated by the machine.
d) Based on your calculations, should the company keep or replace the machine after 10 years?