The diameters of two cones are equal.

It is given that

Diameters of two cones are equal

Therefore their radius are also equal i.e

r1 = r2

Let the ratio of slant height be x

Therefore l1 = 5x

l2 = 4x

Therefore Ratio of curved surface area = c1/c2

$\Rightarrow c_{1} / c_{2}=\pi r_{1} l_{1} / \pi r_{2} l_{2}$

$=\frac{\pi r_{1} * 5 x}{\pi r_{2} * 4 x}$

= 5/4

∴ Ratio of curved surface area is 5: 4.