A plant asset was purchased on January 1 for $80,000 with an estimated salvage value of $20,000 at the end of its useful life. The current year's Depreciation Expense is $10,000 calculated on the straight-line basis and the balance of the Accumulated Depreciation account at the end of the year is $20,000. The remaining useful life of the plant asset is ________ years.
A : 7
B : 5
C : 4
D : 15
How is it calculated?