Question

A company paid ​$150,000 for a new​ 18-wheeler. When it is 9 years old it will be worth ​$42,000. Using​ straight-line depreciation, the value of the truck in​ dollars, V, is a linear function of its age in​ years, n. Find the function and the value when the truck is 4 years old.

Answer 1

Look at it as being given two ordered pairs , (0,150000) and (9,42000)
slope = (42000-150000)/(9-0) = -108000/9 = -12000

so value = V(n) = -12000n + b
but (0,150000) is the "y-intercept"

your function is
V(n) = -12000n + 150000

for the 2nd part, plug in n = 4

Answer 2

V = Vo - Vo*r*T = $42,000.
150,000 - 150,000*r*9 = 42,000.
1 - 9r = 0.28,
9r = 0.72,
r = 0.08/yr. = Annual depreciation rate.

V = Vo - Vo*r*T.
V = 150,000 - 150,000*0.08*4 = $102,000.