c
\(\begin{array}{l} \frac{{dv}}{{dt}} =  - 2.5\sqrt v  \Rightarrow \frac{{dv}}{{\sqrt v }} =  - 2.5dt\\ Integrating,\,\int_{6.25}^0 {{v^{{\raise0.5ex\hbox{\)\scriptstyle { - 1}\(} \kern-0.1em/\kern-0.15em \lower0.25ex\hbox{\)\scriptstyle 2\(}}}}dv =  - 2.5\int_0^t {dt} } \\  \Rightarrow \left[ {\frac{{v{\raise0.5ex\hbox{\)\scriptstyle { + 1}\(} \kern-0.1em/\kern-0.15em \lower0.25ex\hbox{\)\scriptstyle 2\(}}}}{{\left( {{\raise0.5ex\hbox{\)\scriptstyle 1\(} \kern-0.1em/\kern-0.15em \lower0.25ex\hbox{\)\scriptstyle 2\(}}} \right)}}} \right]_{6.25}^0 =  - 2.5\left[ t \right]_0^t\\  \Rightarrow \, - 2{\left( {6.25} \right)^{{\raise0.5ex\hbox{\)\scriptstyle 1\(} \kern-0.1em/\kern-0.15em \lower0.25ex\hbox{\)\scriptstyle 2\(}}}} =  - 2.5t \Rightarrow t = 2\,\sec  \end{array}\)