a plant asset was purchased on january 1 for $180,000 with an estimated residual value of 30,000$ at the end of its useful life. the current year's depreciation expense is $15000 calculated on the straight line basis and the balance of the accumulated depreciation account at the end of the year is 90000$ the remaining useful life of the plant asset is ??

6 or 10 or 4 or 8