Answer:
b. 8 years.
Explanation:
We solve this with the formula for straight line depreciation:
[tex]\frac{cost - salvage }{useful \:\: life}[/tex]
We plug our values and solve
[tex]\frac{40,000 - 8,000 }{useful \:\: life} = 4,000[/tex]
[tex]\frac{32,000}{4,000} = useful \:\: life[/tex]
useful life: 8 years