A demand loan of $$$$8000\mathrm{.}00$ is repaid by payments of $$$$3500\mathrm{.}00$ after two $$years$,$$$3500\mathrm{.}00$ after four $$years$,$ and a final payment after seven years. Interest is $7\%$ compounded semi dash annually for the first two $$years$,8\%$ compounded annually for the next two $$years$,$ and $8\%$ compounded semi dash annually thereafter. What is the size of the final$$ payment?