c
\(\begin{array}{l}
While\,moving\,downhill\,power\\
P = \left( {w\sin \theta  + \frac{w}{{20}}} \right)10\\
P = \left( {\frac{w}{{10}} + \frac{w}{{20}}} \right)10 = \frac{{3w}}{2}\\
\frac{p}{2} = \frac{{3w}}{4} = \left( {\frac{w}{{10}} - \frac{w}{{20}}} \right)V
\end{array}\)

\(\begin{array}{l}
\frac{3}{4} = \frac{v}{{20}} \Rightarrow v = 15m/s\\
\therefore \,Speed\,of\,car\,while\,moving\,downhill\\
v = 15\,m/s.
\end{array}\)