b
\(\begin{array}{l}
{v^2} - {u^2} = 2\,as\\
Give\,v = 20\,m{s^{ - 1}},\,u = 10\,m{s^{ - 1}},\,s = 135\,m\\
\therefore a = \frac{{400 - 100}}{{2 \times 135}} = \frac{{300}}{{270}} = \frac{{10}}{9}m/{s^2}\\
\,\,\,\,\,v = u + at\, \Rightarrow \,t = \frac{{v - u}}{a} = \frac{{10m/s}}{{\frac{{10}}{9}m/{s^2}}} = 9s
\end{array}\)