Preorder traversal of BST $: 15,10,12,11,20,18,16,19$
In Pre-order, the first node is ROOT. So root is $15.$
In Post-order, the last node is ROOT. So in the Post-order sequence, $15$ must be at last.
In Pre-order, the second node is the left child of ROOT, if it is less than the root.
Sequence$: 10,12,11$ belongs to the left sub-tree of ROOT.
$10,12,11$ is a Preorder of BST -- repetitively apply the steps.
In the Pre-order, the nodes which are greater than ROOT are on the right side of ROOT.
Sequence$: 20,18,16,19$ belongs to the right sub-tree of ROOT.
$20,18,16,19$ is a Preorder of BST -- repetitively apply the steps.
Finally we will get $11,12,10,16,19,18,20,15$ as Postorder.