A pipe 20 cm long is closed at one end. Which harmonic mode of the pipe is resonantly excited by a source of 1237.5 Hz? (sound velocity in air = 330 ms-1)

Hint: The frequency of the mode will be equal to the frequency of the source.
                                 
                               Step 1:Findthefundamentalfrequencyofthewave.
Lengthofpipe,l=20cm=20×10-2m
vfundamental=v4L=3304×20×10-2(forclosedpipe)
νfundamental=330×10080=412.5Hz
Step 2:Findthemodeofthepipe.
νgivenνfundamental=1237.5412.5=3
Hence, 3rd harmonic mode of the pipe is resonantly excited by the source of the given frequency.