According to Doppler's effect, Observed frequency of sound reflected from building is given by f′=f(
v+vb
v−vb
)....(i) Here, v= speed of sound =340m/s vb= speed of bus =7.2km/h=20ms−1 and f= original frequency produced by horn =1.7kHz Hence, putting these value in Eq. (i), we get Observed frequency by bus driver,Hence, putting these value in Eq. (i), we get Observed frequency by bus driver,