Problem description here
Each plane can occupy a gate that is equal to or smaller than it. As a result, the most optimal place to put each plane as they come in is the largest unfilled spot that they can reside in (eg. 4 can be in spots 1, 2, 3, 4 so the largest one of those that doesn't already have a plane is the best place to put a plane numbered 4). Do this until it's not possible to fill in planes.