\(\frac{1}{v} + \frac{1}{u} = \frac{1}{f}{\text{ }}\) so \(\frac{{dv}}{{dt}} =  - \frac{{{v^2}}}{{{u^2}}}\left( {\frac{{du}}{{dt}}} \right)\)

\( \Rightarrow \frac{{dv}}{{dt}} =  - {\left( {\frac{f}{{u - f}}} \right)^2}\) \(\frac{{du}}{{dt}} \Rightarrow \frac{{dv}}{{dt}} = \frac{1}{{15}}{\text{m}}/{\text{s}}\)