Answer
(a) The frequency heard is 432 Hz
(b) The friend hears a frequency of 429 Hz
Work Step by Step
(a) We can find the frequency $f'$ heard by the stationary observer.
$f' = \frac{f}{(1-\frac{v_{source}}{v_{snd}})}$
$f' = \frac{400~Hz}{(1-\frac{25.0~m/s}{340~m/s}~)}$
$f' = 432~Hz$
The frequency heard is 432 Hz.
(b) We can find the frequency detected by the person who is approaching.
$f' = (1+\frac{v_{obs}}{v_{snd}})~f$
$f' = (1+\frac{25.0~m/s}{340~m/s})~(400~Hz)$
$f' = 429~Hz$
The friend hears a frequency of 429 Hz.
