Answer
The bat would need to fly away from us with a speed of 86 m/s
Work Step by Step
In order to hear the chirp at a lower frequency, the bat would have to fly away from us. We can find the required speed of the bat.
$f' = \frac{f}{(1+\frac{v_{source}}{v_{snd}})}$
$1+\frac{v_{source}}{v_{snd}} = \frac{f}{f'}$
$\frac{v_{source}}{v_{snd}} = \frac{f}{f'}-1$
$v_{source} = (\frac{f}{f'}-1)~v_{snd}$
$v_{source} = (\frac{25~kHz}{20~kHz}-1)~(343~m/s)$
$v_{source} = 86~m/s$
The bat would need to fly away from us with a speed of 86 m/s.