What are the number of beats heard by the boy per second if he blows a whistle of frequency 680 Hz while moving away from a stationary wall with a speed of 1 m/s?

(Velocity of sound in air = 340 m/s) 

Let n be the actual frequency of source.
For incident wave on the wall:
Boy is source and wall behaves as observer and hears a frequency n₁.

${\mathrm{n}}_1=\frac{\mathrm{C}}{\mathrm{C}-{\mathrm{v}}_{\mathrm{B}}}\mathrm{n}$

For sound wave reflected from the wall
Boy is observer and wall behaves as source of sound with frequency n₁. Now boy observes frequency n₂ of reflected wave.

$\begin{array}{l}{\mathrm{n}}_2=\left(\frac{\mathrm{C}-{\mathrm{v}}_{\mathrm{B}}}{\mathrm{C}}\right){\mathrm{n}}_1\\ \Rightarrow {\mathrm{n}}_2=\left(\frac{\mathrm{C}-{\mathrm{v}}_{\mathrm{B}}}{\mathrm{C}+{\mathrm{v}}_{\mathrm{B}}}\right)\mathrm{n}\\ \Rightarrow {\mathrm{n}}_2=\left(\frac{340-1}{340+1}\right)680=676\mathrm{Hz}\end{array}$

No.of beats heard by boy is,

$\mathrm{\Delta n}={\mathrm{n}}_2~\mathrm{n}\Rightarrow \mathrm{\Delta n}=680-676=4 \mathrm{beats}/\mathrm{s}$