Doppler effect w/ percentage frequency shift?

[grantaere]

Homework Statement

As you stand by a railroad track a train passes with its whistle blowing. As it passes, you hear a frequency shift equal to 21 % of the frequency of the whistle. How fast is the train moving. Express your answer in km/hr.(Speed of sound is 331.4 m/s)

Homework Equations

f’ = f [1/{1 ± (velocity of source/velocity of sound)}]
(source moving away from(+)/towards(-) observer)

The Attempt at a Solution

My initial attempt was simply to have
0.79 = [1/(1 + v of train)/331.4] but that seems to be incorrect... a clue from the teacher was that the train is first moving toward, then away from the observer, but I don't understand how that would affect the formula. Any hints? Thanks a ton in advance!

 

[BvU]

a frequency shift equal to 21 % of the frequency of the whistle

the latter is probably the frequency f₀ when the train is standing still. When the train comes at you you hear f' > f₀ and when it moves away you hear f'' < f₀. Why don't you follow teachers advice and see where it brings you ?