An open pipe is in second harmonic with frequency \( f_1 \). One end of the tube is closed and frequency is increased to \( f_2 \) and it resonates again with nth harmonic. For what value of \( n \), the ratio of \( \frac{f_1}{f_2} \) is \( \frac{4}{5} \)?
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Open pipe supports all harmonics, closed pipe supports only odd harmonics.