A truck with a cost of $123,000 has an estimated residual value of $24,000, has an estimated useful life of 12 years, and is depreciated by the straight-line method. 

(a) Determine the amount of the annual depreciation. 

(b) Determine the book value at the end of the seventh year of use. 

(c) Assuming that at the start of the eighth year the remaining life is estimated to be six years and the residual value is estimated to be $15,000, determine the depreciation expense for each of the remaining six years.