Cant figure out this problem please help and show work thank

Solution

according to doppler effect

f = f0 ( c + vs / c -vr) where c = speed of sound vs = velocity of source towards the reciever

vr = velocity of reciever towards the source

f0 = frequency of sound

f = frequency of sound heard by reciever

f = 30 ( 300 + 60 / 300 - 60) = 45 Hz Answer

now this sound wave is sent back

so reciever becomes source and source becomes reciever

f = 45 (300 + 60 / 300 - 60) = 67.5 Hz Answer