A pipe 20 cm long

Given:

Length of pipe l = 20 cm

Frequency of wave = 1237.5 Hz

We know that the for a pipe closed at one end the resonant frequency of the pipe is given by

where n is the harmonic, v is the speed of the wave and l is the length of the pipe. For first harmonic or fundamental frequency we have n = 1, then

for a frequency of 1237.5 Hz,

Therefore, the given frequency corresponds to the third harmonic.