Answer:

According to the question, the length is:

l = 20 cm =0.2 m

and the frequency is, v = 1237.5 Hz

We know that the velocity of sound in air = 330 m/s

The following expression is:

l = λ/4

λ = 4l

We know that,

v1= v/ λ

v1 = v/4l

v₁ = 412.5 Hz

v/v₁ = 1237.5/412.5

v/v₁ = 3/1

Therefore, the third harmonic will be the one having 1237.5 Hz frequency.