(c) The Doppler's effect in sound is given by fo=
v+vo
v+vs
fs where, fo= observer frequency of sound, v= speed of sound waves, vo= observer velocity, vs= source velocity and fs= actual frequency of sound wave. Thus, according to question, 2n=(
v+0
v+vs
)n ⇒2(v+vs)=v ⇒2v+2vs=v −2vs=v vs=−
v
2
where -ve sign shows that source is approaching to the listener.