A siren emitting a sound of frequency 800 Hz moves away from an...

Given here,

${f}_{o} = 800Hz$
${V}_{s} = 15m{s}^{- 1}$



Using Doppler's effect, the frequency of sound that the observer hears in the echo reflected from the cliff is ${f}_{a} = {f}_{o}\left(\dfrac{V + {V}_{o}}{V - {V}_{s}}\right)$
$⇒ {f}_{a} = \dfrac{330}{\left(330 - 15\right)} \times 800 = \dfrac{330}{315} \times 800$

${f}_{a} = 838Hz$