This is a Doppler effect. Generally, if you move to a frequency source, you would detect an increase in frequency and when you move away from a source you would detect a decrease.
For this question, before you pass them, you are actually approaching them, so you would hear a higher frequency than the constant 300 Hz they are playing at.
Using the condensed formula:
f ' = ((v + vd)/(v + vs)) * f
Where: vd = Velocity of the detector.
vs = Velocity of the frequency source.
v = Velocity of sound in air.
f ' = Apparent frequency.
f = Frequency of source.
v = 343 m/s, vd = detector = 27.8 m/s, vs = velocity of the source =0. (the flautists are not moving).
f = 300 Hz.
There would be an overall increase in frequency, so we maintain a plus at the numerator and a minus at the denominator.
f ' = ((v + vd)/(v - vs)) * f
f ' = ((343+ 27.8)/(343 - 0)) * 300
= (370.8/343)* 300 = 324.3
Therefore frequency before passing them = 324.3 Hz.
Cheers.
