Hi this is mansi. Say answer for this question
In the above question it is not mentioned that wheather the observer is stationary or moving.
Assuming the observer to be stationary and moving away from the observer (since freqency is decreasing)
where f is the frequency of source and v is velocity of sound ans vs is velovity of source
On solving
Study 40% syllabus and score up to 100% marks in JEE