Past solutions don't fully make sense to me. Can it be shown in present value equations?

1.2.8 A manufacturer can automate a certain process by replacing 20 employees with a machine. The employees each earn 24,000 per year, with payments on the last day of each month, with no sala- ry increases scheduled for the next 4 years. If the machine has a lifetime of 4 years and interest is at a monthly rate of .75%, what is the most the manufacturer would pay for the machine (on the first day of a month) in each of the following cases? (a) The machine has no scrap value at the end of 4 years. (b) The machine has scrap value of 200,000 after 4 years. (c) The machine has scrap value of 15% of its purchase price at the end of 4 years