The 17500 Hz whine of the turbines in the jet engines of an aircraft moving with speed 280 m/s is heard at what frequency by the pilot of a second craft trying to overtake the first at a speed of 380 m/s?
At that speed of the second plane, there will be a weak oblique shock wave in front of it. Sound waves that pass through the shock wave will enter a denser slower medium, with a different sound speed. This question is harder than it looks to do correctly.
Neglecting shock wave issues, there will be an upshift in frequency of about (380-280)/340 x 17500 Hz
That will put the received frequency at about 22,650 Hz.
Humans cannot hear that frequency