We give a new constraint qualification which
guarantees strong duality between a cone constrained convex
optimization problem and its Fenchel-Lagrange dual. This result is applied to a convex optimization problem having, for a given non-empty closed convex cone K, as objective function a
K-convex function postcomposed with a K-increasing convex
function. For this so-called composed convex optimization problem we present a strong duality assertion, too, under weaker conditions than the ones considered so far. As application we show that the formula of the conjugate of a postcomposition with a K-increasing convex function is valid under weaker conditions than the ones existing in the literature.