Frequency of sound received at cliff is let $f_1$.

By Doppler's formula, we have

$f_1 =\left(\frac{v}{v-v_s}\right) \cdot f$

$=\frac{v}{v-0.5 v} f=2 f \quad\left[\therefore v_s=0.5 \,v\right]$

Now, reflected sound is observed on a moving engine.

Let observed frequency is $f_2$, then

$f_2=\left(\frac{v-v_0}{v}\right) f_1=\frac{1}{2} f_1=\frac{1}{2} \times 2 f$

$\therefore \quad f_2 \approx 0.990 f$