A tuning fork of frequency 250 Hz is vibrating at one end of a tube as shown in . If maximum sound is 

heard at the other end, the velocity of wave will be

For maximum sound, the  path difference  $\Delta$ Should be equal to $\lambda =v/n$= v/

 path difference x$\mathrm{\Delta x}=\frac{\mathrm{V}}{\mathrm{n}}\text{ or }\mathrm{V}=\mathrm{n\Delta }x$

From figure

. $\mathrm{\Delta x}=(\mathrm{\pi r}-2\mathrm{r})=(3\cdot 14\times 1-2\times 1)=1\cdot 14$
so $V=250\times 1\cdot 14=285m/s$