Answer
$f'=400$ HZ
Work Step by Step
To find the apparent frequency of the tone, use the Doppler effect equation, which is $$f'=f(\frac{v \pm v_o}{v \mp v_s})$$ where $v$ is the speed of sound, $v_o$ is the speed of the observer, and $v_s$ is the speed of the source. Since the observer is moving away from the source, the frequency should sound lower than the true frequency. Therefore, by substituting in values, the apparent frequency is $$f'=(440Hz)(\frac{340m/s-80.0m/s}{340m/s-54.0m/s})=400. Hz$$
