The frequency of whistle of an engine appears to be ${\left(\raisebox{1ex}{$4$}\!\left/ \!\raisebox{-1ex}{$5$}\right.\right)}^{th}$of initial frequency when it crosses a stationary observer. If the velocity of sound is 330 m/s, then the speed of engine will be 

$n\text{'}=\frac{nv}{v-v_s} .....\left(1\right)$

$n"=\frac{nv}{v+v_s} ........\left(2\right)$

From (1) and (2), $\frac{n\text{'}}{n\text{'}\text{'}}=\frac{v+v_s}{v-v_s} .....\left(3\right)$

According to question,$\frac{n\text{'}}{n\text{'}\text{'}}=\frac{5}{4}$

$v_s$ = ? v = 330 m/s                …………….(4)

From eq. (3) and (4)

$\frac{5}{4}=\left[\frac{330+v_s}{330-v_s}\right]$

9$v_s$ = 330

 ∴ $v_s$ = 36.6 m/s