How to use the Doppler effect on this problem?

https://isaacphysics.org/questions/singing_bottles?board=9844fa56-9eab-45cc-b1bc-345316fa67b9&stage=a_level

I am stuck on part d) where I have to find the fractional difference between the frequency of a downward travelling wave and its reflection, as I'm not sure how to find the reflected frequency...

Values I have so far:
Speed of sound = 340
Speed water moves up = 0.038
Frequency of air = 430
Time taken for bottle to fill = 5.3