Answer
$328\;\rm m/s$
Work Step by Step
We can see that the piles of cork particles are the nodes of the sound wave since there is no motion of the air particles there.
This means that the distance between two piles (two nodes) is half wavelength.
$$\lambda =2d$$
where $d$ is the separation distance between two piles.
Hence, the speed of the sound wave is then given by
$$v=\lambda f=2df$$
Plugging the known;
$$v=2(0.41)(400)=\color{red}{\bf 328}\;\rm m/s$$
