The end radii of a frustum are 18 cm and 6 cm, and its vertical height is 16 cm. Find the height of the COMPLETE cone from which it was cut.

Let the small cone removed have height $x$ and the whole cone height $H$. By similar triangles $\frac{6}{18}=\frac{x}{H}$, so $H=3x$. Also $H-x=16$, giving $2x=16,x=8$ and $H=24$ cm. 8 cm is the small cone’s height, not the whole cone’s; 16 cm is the frustum’s own height; 48 cm doubles the correct value.