STANDARD Physics Organ Pipes MCQ Question

For a closed organ pipe, the third harmonic is $\frac{3v}{4l}$. For an open organ pipe, the fundamental frequency is $\frac{v}{2l'}$. Equating these gives $l' = \frac{2 \times 20}{3} = 13.33$ cm. Thus, the closest option is 13.2 cm.