An office machine is purchased for $$8100.$ Under certain assumptions, its salvage value, V $,$ in dollars, is depreciated according to a method called double declining balance, by basically $78\%$ each year, and is given by $V(t)=8100(0\mathrm{.}78{)}^t,$ where $t$ is the time, in years after purchase.

a$)$ Find $V^{\text{'}}(t).$

b$)$ Interpret the meaning of $V^{\text{'}}(t).$

a$)V^{\text{'}}(t)=$