The 7th harmonic of a closed organ pipe has the same frequency as that of the 4th harmonic of an open pipe. If two different gases with the same bulk modulus and ratio of density 1/3 and the length of the closed pipe is 10 cm, find the length of the open pipe.
1
\( \frac{80}{7 \sqrt{3}} \, \text{cm} \)
2
\( \frac{20}{7 \sqrt{3}} \, \text{cm} \)
3
\( \frac{40}{7 \sqrt{3}} \, \text{cm} \)
4
\( \frac{10}{7 \sqrt{3}} \, \text{cm} \)