As source (horn of bus) is approaching stationary wall (say, listener), therefore, apparent frequency striking the wall is v′=
vv
v−vs
.....(1) Sound of this frequency will be reflected by the wall (now, source). The passenger is the listener moving towards source. Therefore, frequency heard by the listener v′′=
(v+vL)v′
v
using (1) v′′=
v+vL
v
×
vv
v−vs
=
(v+vL)v
v−vs
=
(330+5)×200
330−5
=
335
325
×200 v′′=206Hz ∴ Beat frequency =(v′′−v) =206−200=6Hz