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:
[tex] \lambda_3 = \frac{c}{f_3}=\frac{343 m/s}{261.6 Hz} =1.31 m[/tex]
The relationship between length of a closed pipe and wavelength of the standing waves inside is:
[tex] L=\frac{n}{4}\lambda_n [/tex]
where n is the number of the harmonic. In this case, n=3, so the length of the pipe is
[tex] L=\frac{3}{4}(1.31 m)=0.98 m [/tex]
