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?