A bird is sitting on the top of a vertical pole $20 \mathrm{m}$ high and its elevation from a point $O$ on the ground is $45°$. It flies off horizontally straight away from the point $O$. After one second, the elevation of the bird from $O$ is reduced to $30°$. Then the speed (in $\mathrm{m}/\mathrm{s}$) of the bird is