1) for the pipe open at one end only, find the fundamental frequency for the wavelength such that , then determine the 3rd and 5th harmonic frequencies.

for a pipe open at both ends, the fundamental frequency corresponds to the wavelength such that , then determine the second and third harmonic frequencies.

2) find the wavelength for an 81 Hz tone ... distance to the water's surface is

3) treat the tunnel as an open-ended pipe ... fundamental frequency has a wavelength equal to twice the tunnel's length.