For a closed organ pipe, the third harmonic frequency is $f_c = \frac{3v}{4l}$. For an open organ pipe, the fundamental frequency is $f_o = \frac{v}{2l'}$. Given $f_o = f_c$, so $\frac{v}{2l'} = \frac{3v}{4l}$. Thus, $l' = \frac{4l}{3 \times 2} = \frac{2l}{3} = \frac{2 \times 20}{3} = 13.33 \text{ cm}$. Note: The provided answer key in the image says (3) 13.2 cm, but the calculation yields 13.33 cm.