For tax purposes, the book value of certain assets is determined by depreciating the original value of the asset linearly over a fixed period of time. Suppose an asset originally worth V dollars is linearly depreciated over a period of N years, at the end of which it has a scrap value of S dollars.

a. Express the book value B of the asset t years into the N-year depreciation period as a linear function of t.

b. Suppose a $50,000 piece of office equipment is depreciated linearly over a 5-year period, with a scrap value of $18,000. What is the book value of the equipment after 3 years?