$4.$ An excavator has an initial cost of $$150,000$ and an estimated useful life of ten years. The salvage value after ten years of use is estimated to be $$40,000.$

a$.$ What is the book value at the end of the fifth year, if you use the straight$-$line method of depreciation accounting?

b$.$ What is the book value at the end of the fifth year, if you use the sum$-$of$-$the$-$year method of depreciation accounting?

c$.$ What is the book value at the end of the fifth year, if you use the double declining$-$balance method of depreciation accounting?