The wavelength of the third resonance of the closed organ pipe is equal to the ratio between the speed of sound and the frequency of the 3rd harmonic:

The relationship between length of a closed pipe and wavelength of the standing waves inside is:

where n is the number of the harmonic. In this case, n=3, so the length of the pipe is

Answer: I'm not sure what it needs to be rounded to, but I got 37.53501401 m/s
Explanation: The formula for speed is speed = distance/time. You plug in the distance (13.40) and the time (0.357), then divide 13.40 by 0.357
I hope this helps! :)
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