A swimming duck paddles the water with its feet once per time interval of 1.8 s

Question

A swimming duck paddles the water with its feet once per time interval of 1.8 s , producing surface waves with this period. The duck is moving at constant speed in a pond where the speed of surface waves is 0.33 m/s, and the crests of the waves ahead of the duck have a spacing of 0.20 m .
What is the duck's speed?
How far apart are the crests behind the duck?

I'm not sure how to approach this. I tried using the doppler equation with the duck as the source speed and 0.33 m/s as the normal speed, but i did not get the correct answer--i dont think i'm doing this right. any thoughts?

Explanation / Answer

We can use the spacing ahead of the duck to calculate the speed of wave ahead of duck, which is 0.20m / 1.8s = 0.11m/s. So the speed of the duck is 0.33m/s - 0.11m/s = 0.22m/s. And the speed of wave behind the duck is 0.33m/s + 0.22m/s = 0.55m/s. So the spacing is 0.55m/s * 1.8s = 1.00m. Hope it's right.

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