Equipment with a cost of $$702,900$ has an estimated residual value of $$63,900,$ has an estimated useful life of $45$ years, and is depreciated by the straight$-$line method.

a$.$ Determine the amount of the annual depreciation.

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b$.$ Determine the book value after $20$ full years of use.

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c$.$ Assuming that at the start of the year $21$ the remaining life is estimated to be $31$ years and the residual value is estimated to be $$65,500,$ determine the depreciation expense for each of the remaining $31$ years.

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