The difference in frequency in open organ pipe = $\begin{aligned}
& \mathrm{f}=\frac{\mathrm{nv}}{2 \mathrm{L}} \
& \Delta \mathrm{f}=\frac{6 \mathrm{v}}{2 \times 0.6}-\frac{5 \mathrm{v}}{2 \times 0.9} \
& \mathrm{v}=333 \mathrm{m} / \mathrm{s} \
& \Delta \mathrm{f}=740 \mathrm{~Hz}
\end{aligned}$