c
(c) When engine approaches towards observer $n' = n\,\left( {\frac{v}{{v - {v_s}}}} \right)$
when engine going away from observer $n'' = \left( {\frac{v}{{v + {v_S}}}} \right)n$
$\therefore $ $\frac{{n'}}{{n''}} = \frac{{v + {v_s}}}{{v - {v_s}}}$
$\Rightarrow \frac{5}{3} = \frac{{340 + {v_s}}}{{340 - {v_s}}}$
$\Rightarrow {v_s} = 85$$m/s$.