The base of a cone and a cylinder have the same radius 6 cm. They have also the same height 8 cm. The ratio of the curved surface of the cylinder to that of the cone is

$$\eqalign{ & {\text{Slant height of the cone }}\left( l \right) \cr & = \sqrt {{6^2} + {8^2}} \cr & = 10{\text{ cm}} \cr & {\text{Required ratio}} = \frac{{2\pi rh}}{{\pi rl}} \cr & = \frac{{2h}}{l} \cr & = \frac{{2 \times 8}}{{10}} \cr & = 8:5 \cr} $$