This problem simulate an arithmetic series:

End of First year -- 500

End of Second year -- 500 +25

End of Third year -- 500 +50

...

500, 525, 550, ...

\(a_{16} = 500+25(15)\)

\(a_{16} = 875\)

\(s_{16} = \frac{16}{2} (500+875)\)

\(s_{16} = 11~000\)