The ratio of frequencies of the first overtone produced by an open pipe to that of a closed pipe having the same length is

Frequency ratio of first overtones for open and closed pipes

For an open pipe of length $L$, the first overtone (second harmonic) has frequency $f_{\text{open}} = 2 \frac{v}{2L} = \frac{v}{L}$.

For a closed pipe of the same length, the first overtone (third harmonic) has frequency $f_{\text{closed}} = 3 \frac{v}{4L} = \frac{3v}{4L}$.

The ratio is therefore $\frac{f_{\text{open}}}{f_{\text{closed}}} = \frac{v/L}{3v/(4L)} = \frac{1}{3/4} = \frac{4}{3}$.

The frequencies are in the ratio $4:3$.