In Kundt's tube, the distance between two successive nodes is $\frac{\mathit{\lambda }}{\mathit{2}}$, where $\lambda$ is the wavelength of the wave in the tube.

Speed of sound in any medium is 
$v\mathit{=}f\lambda$, where $f$ is the frequency of the wave. 

 

Here, distance between $6$ successive nodes is 
$\frac{\mathit{5}\lambda }{\mathit{2}}=85 \mathrm{cm}$ 
$\lambda =34 \mathrm{cm}=0.34 \mathrm{m}$ 
Now, speed of sound in the tube 
$v=f\lambda$ 
$=1000\times 0.34$ 
$=340 \mathrm{m} {\mathrm{s}}^{-1}$