\[
S'(t) = \frac{3}{2\sqrt{t+1}}, \quad S(15) = 12, \quad S'(15) = \frac{3}{8}, \quad S(16) \approx \frac{99}{8}, \quad S(17) \approx \frac{51}{4}
\]
Thus, the final boxed answers are:
\[
\boxed{S'(t) = \frac{3}{2\sqrt{t+1}}}
\]
\[
\boxed{S(15) = 12}
\]
\[
\boxed{S'(15) = \frac{3}{8}}
\]
\[
\boxed{S(16) \approx \frac{99}{8}}
\]
\[
\boxed{S(17) \approx \frac{51}{4}}
\]