Organ pipe A, with both ends open, has a fundamental frequency of 20 Hz. The third harmonic of organ pipe B, with one end closed has the same frequency as the second harmonic of pipe A. How long is pipe B? Speed of sound in air v_air = 343 m/s.

L_B  = m

Since organ pipe A is open at both ends, the resonant frequencies must satisfy the relationship

where v is the speed of sound in air and L_A is the length of pipe A. For the fundamental frequency n = 1 and the frequency is

Organ pipe B is closed at one end. The resonant frequencies are given by

where v is the speed of sound in air and L_B is the length of pipe B.
We know that