Answer: 394 Hz
Explanation: this is a question under Doppler effect.
The mathematical equation that defines a Doppler effect is given below as
fo = {c+ vo/ c - vs} × f
Where fo = frequency observed =?
c = speed of sound = 345 m/s
vo = velocity of observer relative to source = 9.5m/s
vs = velocity of source relative to observer = - 9.5 m/s ( the negative sign is as a result of the fact that the observer and the source are moving in opposite direction to each other)
f = frequency of sound by source = 394 Hz.
By substituting the parameters, we have that
fo = {345 + 9.5/ 345 - (-9.5)} × 394
fo = (345 +9.5/ 345 + 9.5) × 394
fo = (354.5/354.5) × 394
fo = 1 × 394 = 394 Hz