Question

What is the speed of the observer for whom a note is 10 per cent lower than the emitted frequency ?

Answer 1

As the apparent frequency (v’) is less than emitted frequency (v), the observer must move away from the source. 

If v is the speed of sound and v₀ that of the observer, then

As the apparent frequency is 10% lower than the emitted frequency,

Thus, speed of the observer is (1/10)^th of the speed of sound.