jmc

If the diameter of the cone is $30$ decimeters, then the radius is $r=30/2=15$ decimeters. The height is two times the radius, so the height is $h=30$ decimeters. The volume of the cones is $\frac{1}{3}(\pi r^2)h=\frac{1}{3}(\pi 15^2)\cdot 30=\boxed{2250\pi }$ cubic decimeters.